The Mechanics of Subnetting

Classes of network IP addresses

Classes of IP addresses offer a range from 256 to 16.8 million hosts, as discussed previously in this module. To efficiently manage a limited supply of IP addresses, all classes can be subdivided into smaller subnetworks. Figure provides an overview of the division between networks and hosts
Introduction to and reason for subnetting

To create the subnetwork structure, host bits must be reassigned as network bits. This is often referred to as ‘borrowing’ bits. However, a more accurate term would be ‘lending’ bits. The starting point for this process is always the leftmost host bit, the one closest to the last network octet.

Subnet addresses include the Class A, Class B, and Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion of the major IP address. This is done by assigning bits from the host portion to the original network portion of the address. The ability to divide the original host portion of the address into the new subnet and host fields provides addressing flexibility for the network administrator.

In addition to the need for manageability, subnetting enables the network administrator to provide broadcast containment and low-level security on the LAN. Subnetting provides some security since access to other subnets is only available through the services of a router. Further, access security may be provided through the use of access lists. These lists can permit or deny access to a subnet, based on a variety of criteria, thereby providing more security. Access lists will be studied later in the curriculum. Some owners of Class A and B networks have also discovered that subnetting creates a revenue source for the organization through the leasing or sale of previously unused IP addresses.

A LAN is seen as a single network with no knowledge of the internal network structure. This view of the network keeps the routing tables small and efficient. Given a local node address of 192.168.10.14, the world outside the LAN sees only the advertised major network number of 192.168.10.0. The reason for this is that the local address of 192.168.10.14 is only valid within the LAN 192.168.10.0 and cannot function anywhere else.

Establishing the subnet mask address

Selecting the number of bits to use in the subnet process will depend on the maximum number of hosts required per subnet. An understanding of basic binary math and the position value of the bits in each octet is necessary when calculating the number of subnetworks and hosts created when bits were borrowed. The last two bits in the last octet, regardless of the IP address class, may never be assigned to the subnetwork. These bits are referred to as the last two significant bits. Use of all the available bits to create subnets, except these last two, will result in subnets with only two usable hosts. This is a practical address conservation method for addressing serial router links. However, for a working LAN this would result in prohibitive equipment costs. The subnet mask gives the router the information required to determine in which network and subnet a particular host resides. The subnet mask is created by using binary ones in the host octet or octets. The subnet octet or octets are determined by adding the position value of the bits that were borrowed. If three bits were borrowed, the mask for a Class C address would be 255.255.255.224. This mask may also be represented, in the slash format, as /27. The number following the slash is the total number of bits that were used for the network and subnetwork portion. To determine the number of bits to be used, the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks needed. As an example, the network requires 30 hosts and five subnetworks. A shortcut to determine how many bits to reassign is by using the subnetting chart. By consulting the row titled ”Usable hosts”, the chart indicates that for 30 usable hosts three bits are required. The chart also shows that this creates six usable subnetworks, which will satisfy the requirements of this scheme. The difference between usable hosts and total hosts is a result of using the first available address as the ID and the last available address as the broadcast for each subnetwork. The ability to use these subnetworks is not provided with classful routing. However, classless routing, which will be covered later in the course can recover many of these lost addresses. The method that was used to create the subnet chart can be used to solve all subnetting problems. This method uses the following formula: Number of usable subnets= two to the power of the assigned subnet bits or borrowed bits, minus two (reserved addresses for subnetwork id and subnetwork broadcast) (2 power of borrowed bits) – 2 = usable subnets (23) – 2 = 6 Number of usable hosts= two to the power of the bits remaining, minus two (reserved addresses for subnet id and subnet broadcast) (2 power of remaining host bits) – 2 = usable hosts (25) – 2 = 30

Applying the subnet mask.
Once the subnet mask has been established it then can be used to create the subnet scheme. The chart in the Figure is an example of the subnets and addresses created by assigning three bits to the subnet field. This will create eight subnets with 32 hosts per subnet. Start with zero (0) when numbering subnets. The first subnet is always referenced as the zero subnet.

When filling in the subnet chart three of the fields are automatic, others require some calculation. The subnetwork ID of subnet zero is the same as the major network number, in this case 192.168.10.0. The broadcast ID for the whole network is the largest number possible, in this case 192.168.10.255. The third number that is given is the subnetwork ID for subnet number seven. This number is the three network octets with the subnet mask number inserted in the fourth octet position. Three bits were assigned to the subnet field with a cumulative value of 224. The ID for subnet seven is 192.168.10.224. By inserting these numbers, checkpoints have been established that will verify the accuracy when the chart is completed.

When consulting the subnetting chart or using the formula, the three bits assigned to the subnet field will result in 32 total hosts assigned to each subnet. This information provides the step count for each subnetwork ID. Adding 32 to each preceding number, starting with subnet zero, the ID for each subnet is established. Notice that the subnet ID has all binary 0s in the host portion.

The broadcast field is the last number in each subnetwork, and has all binary ones in the host portion. This address has the ability to broadcast only to the members of a single subnet. Since the subnetwork ID for subnet zero is 192.168.10.0 and there are 32 total hosts the broadcast ID would be 192.168.10.31. Starting at zero the 32nd sequential number is 31. It is important to remember that zero (0) is a real number in the world of networking.

The balance of the broadcast ID column can be filled in using the same process that was used in the subnetwork ID column. Simply add 32 to the preceding broadcast ID of the subnet. Another option is to start at the bottom of this column and work up to the top by subtracting one from the preceding subnetwork ID.

Subnetting Class A and B networks

The Class A and B subnetting procedure is identical to the process for Class C, except there may be significantly more bits involved. The available bits for assignment to the subnet field in a Class A address is 22 bits while a Class B address has 14 bits.

Assigning 12 bits of a Class B address to the subnet field creates a subnet mask of 255.255.255.240 or /28. All eight bits were assigned in the third octet resulting in 255, the total value of all eight bits. Four bits were assigned in the fourth octet resulting in 240. Recall that the slash mask is the sum total of all bits assigned to the subnet field plus the fixed network bits.

Assigning 20 bits of a Class A address to the subnet field creates a subnet mask of 255.255.255.240 or /28. All eight bits of the second and third octets were assigned to the subnet field and four bits from the fourth octet.

In this situation, it is apparent that the subnet mask for the Class A and Class B addresses appear identical. Unless the mask is related to a network address it is not possible to decipher how many bits were assigned to the subnet field.

Whichever class of address needs to be subnetted, the following rules are the same:

Total subnets = 2 ^{to the power of the bits borrowed}
Total hosts= 2 ^{to the power of the bits remaining}
Usable subnets = 2 ^{to the power of the bits borrowed} minus 2
Usable hosts= 2 ^{to the power of the bits remaining} minus 2

Calculating the resident subnetwork through ANDing.

Routers use subnet masks to determine the home subnetwork for individual nodes. This process is referred to as logical ANDing. ANDing is a binary process by which the router calculates the subnetwork ID for an incoming packet. ANDing is similar to multiplication. This process is handled at the binary level. Therefore, it is necessary to view the IP address and mask in binary. The IP address and the subnetwork address are ANDed with the result being the subnetwork ID. The router then uses that information to forward the packet across the correct interface. Subnetting is a learned skill. It will take many hours performing practice exercises to gain a development of flexible and workable schemes. A variety of subnet calculators are available on the web. However, a network administrator must know how to manually calculate subnets in order to effectively design the network scheme and assure the validity of the results from a subnet calculator. The subnet calculator will not provide the initial scheme, only the final addressing. Also, no calculators, of any kind, are permitted during the certification exam.

Cisco Systems, Inc.