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Binary patterns representing IPv4 addresses are expressed as dotted decimals by separating each byte of the binary pattern, called an octet, with a dot. Note: Calculating the number of hosts and determining which portion of the 32 bits refers to the network will be covered later in this chapter. To understand the operation of a device in a network, we need to look at addresses and other data the way the device does - in binary notation. Learning to convert binary to decimal requires an understanding of the mathematical basis of a numbering system called positional notation.
When we interpret a byte as a decimal number, we have the quantity that position represents if the digit is a 1 and we do not have that quantity if the digit is a 0. Not only do we need to be able to convert binary to decimal, we also need to be able to convert decimal to binary. Because our representation of addresses is limited to decimal values for a single octet, we will only examine the process of converting 8-bit binary to the decimal values of 0 to 255.
Within the IPv4 address range of a network, the lowest address is reserved for the network address.
The IPv4 broadcast address is a special address for each network that allows communication to all the hosts in that network.
Because broadcast traffic is used to send packets to all hosts in the network, a packet uses a special broadcast address. When a host needs information, the host sends a request, called a query, to the broadcast address. The limited broadcast is used for communication that is limited to the hosts on the local network.
As you learned earlier, when a packet is broadcast, it uses resources on the network and also forces every host on the network that receives it to process the packet. Although most IPv4 host addresses are public addresses designated for use in networks that are accessible on the Internet, there are blocks of addresses that are used in networks that require limited or no Internet access. Private space address blocks, as shown in the figure, are set aside for use in private networks. With services to translate private addresses to public addresses, hosts on a privately addressed network can have access to resources across the Internet. As explained earlier, within each network the first and last addresses cannot be assigned to hosts.
Historically, RFC1700 grouped the unicast ranges into specific sizes called class A, class B, and class C addresses.
Class B address space was designed to support the needs of moderate to large size networks with more than 65,000 hosts. The allocation of Network layer address space within the corporate network needs to be well designed. Similarly, we need to monitor the security and performance of the network hosts and the network as a whole. As you have already learned, hosts are associated with an IPv4 network by a common network portion of the address. Each of these different device types should be allocated to a logical block of addresses within the address range of the network.
An important part of planning an IPv4 addressing scheme is deciding when private addresses are to be used and where they are to be applied.
If there are more devices than available public addresses, only those devices that will directly access the Internet - such as web servers - require a public address. In most data networks, the largest population of hosts includes the end devices such as PCs, IP phones, printers, and PDAs. With a static assignment, the network administrator must manually configure the network information for a host, as shown in the figure.
Because of the challenges associated with static address management, end user devices often have addresses dynamically assigned, using Dynamic Host Configuration Protocol (DHCP), as shown in the figure.
DHCP enables the automatic assignment of addressing information such as IP address, subnet mask, default gateway, and other configuration information.
DHCP is generally the preferred method of assigning IP addresses to hosts on large networks because it reduces the burden on network support staff and virtually eliminates entry errors.
Any network resource such as a server or a printer should have a static IPv4 address, as shown in the figure. In most internetworks, only a few devices are accessible by hosts outside of the corporation. Unlike the other intermediary devices mentioned, routers and firewall devices have an IPv4 address assigned to each interface.
A company or organization that wishes to have network hosts accessible from the Internet must have a block of public addresses assigned. To get access to the services of the Internet, we have to connect our data network to the Internet using an Internet Service Provider (ISP).
ISPs have their own set of internal data networks to manage Internet connectivity and to provide related services.
ISPs are designated by a hierarchy based on their level of connectivity to the Internet backbone.
In the early 1990s, the Internet Engineering Task Force (IETF) grew concerned about the exhaustion of the IPv4 network addresses and began to look for a replacement for this protocol.
Because the high order bits of the subnet masks are contiguous 1s, there are only a limited number of subnet values within an octet.
If the subnet mask for an octet is represented by 255, then all the equivalent bits in that octet of the address are network bits. Inside data network devices, digital logic is applied for their interpretation of the addresses. This ANDing between the host address and subnet mask is performed by devices in a data network for various reasons. To calculate the number of hosts per network, we use the formula of 2^n - 2 where n = the number of bits left for hosts. Applying this formula, (2^7 - 2 = 126) shows that each of these subnets can have 126 hosts. Every network within the internetwork of a corporation or organization is designed to accommodate a finite number of hosts. Next, consider the number of networks and the size of each required based on common groupings of hosts. Now that we have a count of the networks and the number of hosts for each network, we need to start allocating addresses from our overall block of addresses. In our example, we now allocate blocks of addresses to the four locations as well as the WAN links. With the major blocks allocated, next we subnet any of the locations that require dividing.
Recall from the previous section that as we divide the address range into subnets, we lose two host addresses for each new network. Recall in previous examples we began with the original subnets and gained additional, smaller, subnets to use for the WAN links. Once you have downloaded your image, you have life-long rights to use it under the terms of the license purchased.
At the Network layer, the packets of the communication need to be identified with the source and destination addresses of the two end systems.
The dotted decimal format is used to make it easier for people to use and remember addresses. At Layer 3, we define a network as a group of hosts that have identical bit patterns in the network address portion of their addresses. Positional notation means that a digit represents different values depending on the position it occupies. To begin the conversion process, we start by determining if the decimal number is equal to or greater than our largest decimal value represented by the most-significant bit.
To send data to all hosts in a network, a host can send a single packet that is addressed to the broadcast address of the network.
In IPv4 addresses, we assign the values between the network address and the broadcast address to the devices in that network. In all three cases, the IPv4 address of the originating host is placed in the packet header as the source address. Unicast packets use the host address of the destination device as the destination address and can be routed through an internetwork.
When a host receives a packet with the broadcast address as the destination, it processes the packet as it would a packet to its unicast address. Therefore, broadcast traffic should be limited so that it does not adversely affect performance of the network or devices. The multicast clients use services initiated by a client program to subscribe to the multicast group. As you have already seen, not all of these addresses can be used as host addresses for unicast communication.
Additionally, the multicast address range is subdivided into different types of addresses: reserved link local addresses and globally scoped addresses.
These services, called Network Address Translation (NAT), can be implemented on a device at the edge of the private network. These addresses are designed to be used in the hosts that are publicly accessible from the Internet.
There are also special addresses that can be assigned to hosts but with restrictions on how those hosts can interact within the network.
These addresses can be automatically assigned to the local host by the operating system in environments where no IP configuration is available. Classful allocation of address space often wasted many addresses, which exhausted the availability of IPv4 addresses. With the classless system, address blocks appropriate to the number of hosts are assigned to companies or organizations without regard to the unicast class.
Without the proper planning and documentation of these network allocations, we could easily assign an address to more than one host.
As part of the monitoring process, we examine network traffic looking for addresses that are generating or receiving excessive packets. A NAT service would allow those devices with private addresses to effectively share the remaining public addresses.
Because this population represents the largest number of devices within a network, the largest number of addresses should be allocated to these hosts.
At a minimum, this includes entering the host IP address, subnet mask, and default gateway. The configuration of the DHCP server requires that a block of addresses, called an address pool, be defined to be assigned to the DHCP clients on a network.

If the host is powered down or taken off the network, the address is returned to the pool for reuse. Each interface is in a different network and serves as the gateway for the hosts in that network. The use of these public addresses is regulated and the company or organization must have a block of addresses allocated to it. An ISP will generally supply a small number of usable IPv4 addresses (6 or 14) to their customers as a part of their services. Among the other services that an ISP generally provides to its customers are DNS services, e-mail services, and a website.
Each lower tier obtains connectivity to the backbone via a connection to a higher tier ISP, as shown in the figure. These ISPs are large national or international ISPs that are directly connected to the Internet backbone. New protocols at various layers of the stack have been developed to support this new protocol. We referred to the prefix length as the number of bits in the address giving us the network portion.
The remaining bits (low order) of the subnet mask are zeroes, indicating the host address within the network.
You will recall that we only need to expand an octet if the network and host division falls within that octet.
Similarly, if the subnet mask for an octet is represented by 0, then all the equivalent bits in that octet of the address are host bits.
When an IPv4 packet is created or forwarded, the destination network address must be extracted from the destination address. Since we use a router to connect these networks together, each interface on a router must have a unique network ID.
We must use a block of addresses that is large enough to accommodate all devices in all the corporate networks. When identifying the total number of hosts using traditional subnetting, we allocate the same number of addresses for each subnet. By creating smaller subnets, each subnet is able to support 2 hosts, leaving the original subnets free to be allotted to other devices and preventing many addresses from being wasted. We will consider subnetting based on the number of hosts, including router interfaces and WAN connections. With IPv4, this means that each packet has a 32-bit source address and a 32-bit destination address in the Layer 3 header.
Although all 32 bits define the IPv4 host address, we have a variable number of bits that are called the host portion of the address. Data represented in binary may represent many different forms of data to the human network.
More specifically, the value that a digit represents is that value multiplied by the power of the base, or radix, represented by the position the digit occupies.
In the example network divisions, we need to look at the octet of the address where the prefix divides the network portion from the host portion. One or more of the hosts with the requested information will respond, typically using unicast. To reach multiple destination hosts using unicast communication, a source host would need to send an individual packet addressed to each host.
One additional type of multicast address is the administratively scoped addresses, also called limited scope addresses.
Although we have previously covered some of these addresses, the major reserved addresses are discussed in the next section.
Hosts that do not require access to the Internet at large may make unrestricted use of private addresses. Even within these address blocks, there are many addresses that are designated for other special purposes. These might be used in a small peer-to-peer network or for a host that could not automatically obtain an address from a Dynamic Host Configuration Protocol (DHCP) server. Unlike the experimental addresses, network devices will accept these addresses in their configurations.
The unicast address classes A, B, and C defined specifically-sized networks as well as specific address blocks for these networks, as shown in the figure.
For example, a company that had a network with 260 hosts would need to be given a class B address with more than 65,000 addresses.
If we have proper planning and documentation of the network addressing, we can identify the device on the network that has a problematic address. Addresses assigned to this pool should be planned so that they exclude any addresses used for the other types of devices. As with all devices in a network that provide network resources, the IPv4 addresses for these devices should be static. Therefore, these network devices provide an opportune location for network management, monitoring, and security. Depending on the level of service required and available, customers use different tiers of an ISP. The customers of Tier 1 ISPs are either lower-tiered ISPs or large companies and organizations.
Creating expanded addressing capabilities was the initial motivation for developing this new protocol. Because of better tools, technologies, and address management in the last few years, IPv4 is still very widely used, and likely to remain so for some time into the future. The subnet mask is configured on a host in conjunction with the IPv4 address to define the network portion of that address. In each of these cases, it is not necessary to expand this octet to binary to determine the network and host portions. The router checks the destination address and attempts to associate this address with a next hop. When connecting the different locations, we use a router to account for the hardware differences between the LANs and the WAN. We start with the locations that require the most hosts and work down to the point-to-point links. If all the subnets have the same requirements for the number hosts, these fixed size address blocks would be efficient. Although we have accomplished the task of dividing the network into an adequate number of networks, it was done with a significant waste of unused addresses. The number of bits used in this host portion determines the number of hosts that we can have within the network.
If the value is smaller than 128, we place a 0 in the 128-bit position and move to the 64-bit position.
Similarly, when a host needs to send information to the hosts on a network, it creates and sends a broadcast packet with the information. With multicast, the source host can send a single packet that can reach thousands of destination hosts. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address as well as packets addressed to its uniquely allocated unicast address.
However, the internal networks still must design network address schemes to ensure that the hosts in the private networks use IP addresses that are unique within their networking environment. While there are some limitations and performance issues with NAT, clients for most applications can access services over the Internet without noticeable problems. Communication using IPv4 link-local addresses is only suitable for communication with other devices connected to the same network, as shown in the figure. A company or organization was assigned an entire class A, class B, or class C address block.
To reserve address space for the remaining address classes, all class A addresses required that the most significant bit of the high-order octet be a zero.
This meant that a class C network used only the last octet as host addresses with the three high-order octets used to indicate the network address. Even though this classful system was all but abandoned in the late 1990s, you will see remnants of it in networks today.
If the addresses for these resources are not planned and documented, the security and accessibility of the devices are not easily controlled.
For instance, they are useful for printers, servers, and other networking devices that need to be accessible to clients on the network.
In the case of servers accessible by the Internet, each of these must have a public space address associated with it. However, if we need to access these devices as hosts to configure, monitor, or troubleshoot network operation, they need to have addresses assigned.
This assignment should be uniform across all networks in the corporation so that network personnel will always know the gateway of the network no matter which network they are working on. Because they are at the top of Internet connectivity, they engineer highly reliable connections and services. These tier 2 ISPs tend to have the IT resources to operate their own services such as DNS, e-mail servers, and web servers. Because of the increased size of the IPv6 header, it also impacts the underlying network infrastructure. The IPv4 host address is logically ANDed with its subnet mask to determine the network address to which the host is associated. As a packet arrives at a router, the router performs ANDing on the IP destination address in the incoming packet and with the subnet mask of potential routes. Network administrators need to devise the internetwork addressing scheme to accommodate the maximum number of hosts for each network.
Consider the example of a corporate internetwork that needs to accommodate 800 hosts in its four locations.
Although hosts in a common geographic location typically comprise a single block of addresses, we may need to subnet this block to form additional networks at each location. This process ensures that large enough blocks of addresses are made available to accommodate the hosts and networks for these locations.
As with any subnetting, we need to carefully plan the address allocation so that we have available blocks of addresses. For example, if we need to have at least 200 hosts in a particular network, we would need to use enough bits in the host portion to be able to represent at least 200 different bit patterns. For the decimal number 245, the value that the 2 represents is 2*10^2 (2 times 10 to the power of 2). This process of extracting the network address from a host address will be explained later. If the value in the 128-bit position is larger than or equal to 128, we place a 1 in the 128 position and subtract 128 from the number being converted.

The scope of multicast traffic also may be limited to the local network or routed through an internetwork.
Unlike unicast, where the packets can be routed throughout the internetwork, broadcast packets are usually restricted to the local network.
Although routers do not forward directed broadcasts by default, they may be configured to do so.
A host must not send a packet with an IPv4 link-local destination address to any router for forwarding and should set the IPv4 TTL for these packets to 1.
Class C address blocks set aside address space for class D (multicast) and class E (experimental) by using a fixed value of 110 for the three most significant bits of the high-order octet.
For example, when you assign an IPv4 address to a computer, the operating system examines the address being assigned to determine if this address is a class A, class B, or class C.
Additionally, variations in the address of one of these devices will make this device inaccessible from the Internet. Because we need to know how to communicate with intermediary devices, they should have predictable addresses.
Router and firewall interfaces are the concentration point for traffic entering and leaving the network. If we choose to move our Internet connectivity to another ISP, the new ISP will provide us with addresses from the address blocks that have been provided to them, and our previous ISP returns the blocks loaned to us to their allocation to be loaned to another customer. Among the technologies used to support this reliability are multiple connections to the Internet backbone.
To define the network and host portions of an address, the devices use a separate 32-bit pattern called a subnet mask, as shown in the figure.
When this ANDing between the address and the subnet mask is performed, the result yields the network address.
Because all the bits of the subnet mask that represent host bits are 0s, the host portion of the resulting network address becomes all 0s. This yields a network address that is compared to the route from the routing table whose subnet mask was used.
Due to improper configuration, a host may see itself on a network that was not the intended one.
This is done by extending the mask to borrow some of the bits from the host portion of the address to create additional network bits.
We need to create subnetworks at the different locations that have hosts for common user needs.
When making the divisions and assignment of available subnets, make sure that there are adequately-sized address blocks available for the larger demands. The creation of new, smaller networks from a given address block is done by extending the length of the prefix; that is, adding 1s to the subnet mask. For example, the topology in Figure 1 shows a subnet requirement of seven subnets, one for each of the four LANs and one for each of the three WANs.
There are 28 unused addresses in each of the three WAN subnets that have been locked into these address blocks. To provide address blocks for the WANS with two addresses each, we will borrow three additional host bits to be used as network bits. For us in the human network, a string of 32 bits is difficult to interpret and even more difficult to remember. Later in this chapter, we will learn more about another entity that is used to specify the network portion of an IPv4 address to the network devices. To get started understanding this process of determining the address assignments, let's break some examples down into binary. In an IPv4 network, the unicast address applied to an end device is referred to as the host address.
This restriction is dependent on the configuration of the router that borders the network and the type of broadcast. Packets using these addresses as the source or destination should not appear on the public Internet. Even though the class A addresses reserved one-half of the address space, because of their limit of 128 networks, they could only be allocated to approximately 120 companies or organizations. The operating system then assumes the prefix used by that class and makes the appropriate subnet mask assignment. When monitoring network traffic with a tool like Wireshark, a network administrator should be able to rapidly identify these devices. Because the hosts in each network use a router or firewall device interface as the gateway out of the network, many packets flow through these interfaces. The primary disadvantage of Tier 2 ISPs, as compared to Tier 1 ISPs, is slower Internet access. Tier 3 ISPs often bundle Internet connectivity as a part of network and computer service contracts for their customers. Recall that an IPv4 address with all 0s in the host portion represents the network address. An originating host must determine if a packet should be sent directly to a host in the local network or be directed to the gateway.
This can create an operation that seems erratic unless diagnosed by examining the ANDing processes used by the host. We may also have other groups of users that require many network resources, or we may have many users that require their own subnetwork. Also, plan carefully to ensure that the address blocks assigned to the subnet do not overlap.
Doing this allocates more bits to the network portion of the address to provide more patterns for the new subnet. Positional notation refers to this position as the base^2 position because the base, or radix, is 10 and the power is 2. For unicast communication, the host addresses assigned to the two end devices are used as the source and destination IPv4 addresses. The router or firewall device at the perimeter of these private networks must block or translate these addresses. Class B had slightly more efficient allocation of addresses than class A because it equally divided 25% of the total IPv4 address space among approximately 16,000 networks.
Although it occupied only 12.5% of the total IPv4 address space, it could provide addresses to 2 million networks.
This means that the router or firewall at the perimeter of the network must be configured to translate the internal address of the server into a public address. Additionally, the addresses of these devices should be in a different range within the network block than user device addresses. At that time, the remaining IPv4 address space was allocated to various other registries to manage for particular purposes or for regional areas. Because these customers are only one connection away from the Internet, there are fewer opportunities for failures or traffic bottlenecks.
Because Tier 2 ISPs are at least one more connection away from the Internet backbone, they also tend to have lower reliability than Tier 1 ISPs. While they may have reduced bandwidth and less reliability than Tier 1 and Tier 2 providers, they are often good choices for small to medium size companies.
The subnet mask is created by placing a binary 1 in each bit position that represents the network portion and placing a binary 0 in each bit position that represents the host portion.
Also, a router may have many different routes that can satisfy the forwarding of packet to a given destination.
This would mean that the bits for the upper three octets would represent the network portion.
The subnet mask consists of 32 bits, just as the address does, and uses 1s and 0s to indicate which bits of the address are network bits and which bits are host bits. During the encapsulation process, the source host places its IPv4 address in the unicast packet header as the source host address and the IPv4 address of the destination host in the packet header as the destination address.
Even if these packets were to make their way to the Internet, the routers would not have routes to forward them to the appropriate private network.
When some routing protocols receive an advertised route, it may assume the prefix length based on the class of the address. When using static IP addressing, it is necessary to maintain an accurate list of the IP address assigned to each device. Because of this additional configuration in the perimeter intermediary device, it is even more important that these devices have a predictable address. Grouping the different types of devices into logical addressing groups makes the assignment and operation of this packet filtering more efficient. These registration companies are called Regional Internet Registries (RIRs), as shown in the figure. The prefix and the subnet mask are different ways of representing the same thing - the network portion of an address.
When each of these 1s is ANDed with the corresponding bit of the address, the resulting bits are identical to the original address bits.
For example, if we use 1 bit, we have the potential to divide that block into two smaller networks.
The communication using a unicast packet can be forwarded through an internetwork using the same addresses. A logical AND is also performed by an originating host between the destination address of the packet and the subnet mask of the this host.
For example, the prefix forming these routes is not directly associated with the networks assigned to the host. Having a different prefix number changes the host range and broadcast address for each network. If this network address matches the network address of the local host, the packet is sent directly to the destination host. If there were issues with routing packets, you would need to determine how the router would make the routing decision. Notice that the network address could remain the same, but the host range and the broadcast address are different for the different prefix lengths. Although there are subnet calculators available, it is helpful for a network administrator to know how to manually calculate subnets. In this figure you can also see that the number of hosts that can be addressed on the network changes as well.
Although for this example we expanded all of the octets, we only need to examine the content of the divided octet.

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