Cisco Basic IP Addressing

An IP address is a unique 4-octet (32-bit) value expressed in dotted-decimal (or dotted-quad) notation of the form W.X.Y.Z, where periods (dots) are used to separate each of the 4 octets of the address (for example, 10.0.0.1). The 32-bit address field consists of two parts: a network or link number (which represents the network portion of the address) and a host number (which identifies a host on the network segment).
The network and host boundaries were traditionally defined based on the class of the IP address, with five defined classes (three of which are used for unicast addressing): A, B, C, D, and E.

Table 3-1 illustrates the different classes of address space and their functions.

Notice that only Class A, B, and C addresses are used for unicast. Class D addresses are used for multicast, and Class E address space is reserved. Several addresses within these classes are reserved for special use.

Table 3-2 lists some of these addresses.

This class-based addressing scheme is often referred to as the classful model. The different classes lend themselves to different network configurations, depending on the desired ratio of networks to hosts. The full implications of the different classes will become more apparent as this chapter proceeds. The next few sections focus on the basic definitions of each class.
Class A Addressing
Class A networks are represented by a 0 in the leftmost bit position of the address. The first octet (bits 0 to 7) of the address, beginning from the leftmost bit, represents the network number, and the remaining 3 octets (bits 8 to 31) represent a host number on that network. An example of a Class A network is 124.0.0.1, where 124.0.0.0 represents the network number and the host number is 1. The outcome of this representation, illustrated in Figure 3-1, is
128(27) Class A network numbers. However, because 0.0.0.0 is not a valid network number,only 127(27-1) Class A addresses are possible.
Figure 3-1. General Class A Address Format

After the network is defined, the first and last host addresses within the network serve special functions. The first address (124.0.0.0 in the previous example) is used to represent the network number, and the last address of the network is used to represent the directed broadcast address of the network (124.255.255.255). Therefore, Class A addresses have only 16,777,214 (224-2) hosts per network, rather than 16,777,216 (224) hosts per network.
Class B Addressing
Class B networks are represented by a 1 and a 0 in the leftmost two bits of the address. The first two octets of the address (bits 0 to 15) represent the network portion of the address, and the remaining two octets (bits 16 to 31) represent the host number of that network. The outcome of this representation, illustrated in Figure 3-2, is 16,384 (214) network numbers,with 65,534 (216-2) hosts per network. An example of a Class B address is 172.16.0.1, where 172.16.0.0 is the Class B network and 1 is the host.
Figure 3-2. General Class B Address Format

Class C Addressing
Class C networks are represented by 1, 1, and 0 in the leftmost three bits of the address. The first three octets (bits 0 to 23) represent the network number, and the last octet (bits 24 to 31) represents the host number in that network. The outcome of this representation, as illustrated in Figure 3-3, is 2,097,152 (221) network numbers with 254 (28-2) hosts per network. An example of a Class C network is 192.11.1.1, where 192.11.1.0 is the network number and the host number is 1.
Figure 3-3. General Class C Address Format

Class D Addressing
Class D networks are represented by 1, 1, 1, and 0 in the leftmost 4 bits of the address. The Class D address space is reserved for multicast, used to represent multicast group numbers.
Class E Addressing
Class E networks are represented by 1, 1, 1, and 1 in the leftmost 4 bits of the address. Class E address space is currently reserved for experimental use.
Basic IP Subnetting
Basic subnetting and variable-length subnets are often misunderstood. This section gives a brief introduction to how subnetting works, and the next section discusses variable-length subnet masks (VLSMs).
A subnet, or subnetwork, is a subset of a Class A, B, or C network. To better understand subnetting, it helps to take a closer look at IP addresses that are not subnetted. As explained earlier, IP addresses are comprised of a network portion and a host portion, representing a static two-level hierarchical addressing model (networks and hosts). IP subnetting introduces a third level of hierarchy with the concept of a network mask, or netmask. The netmask serves
as a bitmask with bits set corresponding to the bits used to the classful IP network number, as well as additional bits set corresponding to the subnet number.
In Figure 3-4, the network mask 255.0.0.0 is applied to network 10.0.0.0. The mask in binary notation is a series of contiguous 1s followed by a series of contiguous 0s. The 1s portion represents the network portion of the address, and the 0s represent the host portion. This provides a mechanism to split the IP address of host 10.0.0.1 into a network portion of 10 and a host portion of 1.
Figure 3-4. Basic Network Masking

Class A, B, and C addresses each have what is referred to as a natural mask, which is the mask created by the very definition of the network and host portions of each class. The natural masks for Class A, B, and C addresses are as follows:
Class A natural mask is 255.0.0.0
Class B natural mask is 255.255.0.0
Class C natural mask is 255.255.255.0
By separating the network and host portions of the IP address, masks facilitate the creation of subnets. Without subnets, network numbers would be very limited in use. Each physical segment, such as an Ethernet, Token Ring, or FDDI segment, is normally associated with one or more network numbers. If subnetting were not available, a Class A network of the form 10.0.0.0 would accommodate only one physical segment with about 16 million hosts on it, as
shown in Figure 3-5.
Figure 3-5. Unsubnetted Class A Address Space

With the use of masks, networks can be divided into smaller subnetworks by extending the network portion of the address into the host portion. The subnetting technique provides a larger number of subnetworks while reducing the number of hosts on each network.
In Figure 3-6, a mask of 255.255.0.0 is applied to network 10.0.0.0. This divides the IP address 10.0.0.1 into a network portion of 10, a subnet portion of 0, and a host portion of 1.
The 255.255.0.0 mask has borrowed a portion of the host space and has applied it to the network space. As a result, the classful ten-network space has increased from a single large network to 256 subnetworks ranging from 10.0.0.0 to 10.255.0.0. This would decrease the number of hosts per each subnet from 16,777,214 to 65,534.
Figure 3-6. Basic Subnetting

NOTE
Note that in this example, 10.0.0.0 represents the zero subnet. Some legacy router software does not allow the zero subnet address space to be used, nor is it used by default in Cisco routers. In order to enable the use of zero subnets in IOS, you must configure ip subnet-zero.

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