Chapter 5 Addressing Dr. Clincy Lecture

Subnetting Dr. Clincy Lecture

SUBNETTING When we talked about CLASSFUL addressing – we realized the problem of wasted host addresses and depleting available network addresses. In subnetting, a network is divided into several smaller networks called subnetworks or subnets – each subnet will have it’s own address Typically, there are 2 steps in reaching a destination: first we must reach the network (netid) and then we reach the destination (hostid) Dr. Clincy Lecture

A network with two levels of hierarchy (not subnetted) The 2 level approach is not enough some times – you can only have 1 physical network – in example, all host are at the same level – no grouping Dr. Clincy Lecture

A network with three levels of hierarchy (subnetted) (0-63) (64-127) With subnetting, hosts can be grouped (128-191) (192-255) Dr. Clincy Lecture

Addresses in a network with and without subnetting With subnetting, there are 3 levels (versus 2 levels). Partition the hostid space into subnetid and hostid. (1st) network, (2nd) subnetwork and (3rd) host Dr. Clincy Lecture

Similar to Hierarchy concept in a telephone number Dr. Clincy Lecture

Default mask and subnet mask Dr. Clincy Lecture

Finding the Subnet Address Given an IP address, we can find the subnet address the same way we found the network address in the previous chapter. We apply the mask to the address. We can do this in two ways: straight or short-cut. Straight Method In the straight method, we use binary notation for both the address and the mask and then apply the AND operation to find the subnet address. Short-Cut Method ** If the byte in the mask is 255, copy the byte in the address. ** If the byte in the mask is 0, replace the byte in the address with 0. ** If the byte in the mask is neither 255 nor 0, we write the mask and the address in binary and apply the AND operation. Dr. Clincy Lecture

Subnet Mask Form In the early days, non-contiguous 1’s masks were used (0’s and 1’s could alternate) Today, as a best practice, contiguous 1’s masks are used In either case, the black box can perform the “masking” process Dr. Clincy Lecture

Example 1 What is the subnetwork address if the destination address is 200.45.34.56 and the subnet mask is 255.255.240.0? Solution 11001000 00101101 00100010 00111000 11111111 11111111 11110000 00000000 11001000 00101101 00100000 00000000 The subnetwork address is 200.45.32.0. Dr. Clincy Lecture

Recall - 5-bit Address Space Illustration 1-bit Netid case (no subnets) 16 addresses/block Number of blocks: 2 Address range per block: 0 to 15 Netids: 0, 1 Network Addresses : 00000, 10000 Broadcast Addresses: 01111, 11111 Dr. Clincy Lecture

5-bit Address Space Illustration subnet 1-bit Subnet case Number of blocks/networks: 2 Number subnets per block: 2 8 addresses/subnet Address range per subnet: 0 to 7 Subnet ids: 0, 1 Network Addresses : 00000, 01000, 10000, 11000 Broadcast Addresses: 00111, 01111, 10111, 11111 Dr. Clincy Lecture

5-bit Address Space Illustration subnet 2-bit Subnet case Number of blocks/networks: 2 Number subnets per block: 4 4 addresses/subnet Address range per subnet: 0 to 3 Subnet ids: 00, 01, 10, 11 Network Addresses : 00000, 00100, 01000, 01100 10000, 10100, 11000, 11100 Broadcast Addresses: 00011, 00111, 01011, 01111 10011, 10111, 11011, 11111 Dr. Clincy Lecture

Example 2 What is the subnetwork address if the destination address is 19.30.84.5 and the mask is 255.255.192.0? Dr. Clincy Lecture

Comparison of a default mask and a subnet mask A portion of the hostid space is divided between some contiguous 1’s and 0’s Dr. Clincy Lecture

The number of subnets must be a power of 2. Determine the number of subnets added by looking at the number of 1s added to the default mask and performing 2 raised to that number For example, 23 = 8 subnets Dr. Clincy Lecture

Example 3 A company is granted the site address 201.70.64.0 (class C). The company needs six subnets. Design the subnets. Solution The number of 1s in the default mask is 24 (class C). The company needs six subnets. This number 6 is not a power of 2. The next number that is a power of 2 is 8 (23). We need 3 more 1s in the subnet mask. The total number of 1s in the subnet mask is 27 (24 + 3). The total number of 0s is 5 (32 - 27). The mask would be Dr. Clincy Lecture

The number of subnets is 8. Solution (Continued) 11111111 11111111 11111111 11100000 or 255.255.255.224 The number of subnets is 8. The number of addresses in each subnet is 25 (5 is the number of 0s) or 32. Dr. Clincy Lecture

Example 3 Dr. Clincy Lecture