Content

| 4.1 |  | RIP v2 Overview |
| 4.1.6 |  | Classless routing protocols |

The true defining characteristic of classless routing protocols is the capability to carry subnet masks in their route advertisements. One benefit of having a mask associated with each route is that the all-zeros and all-ones subnets are now available for use. Classful routing protocols cannot distinguish between an all-zeros subnet, for example 172.16.0.0, and the major network number 172.16.0.0. Likewise, they cannot distinguish between a broadcast on the all-ones subnet 172.16.255.255, and an all-subnets broadcast 172.16.255.255.

If the subnet masks are included, this difficulty disappears. In this example 172.16.0.0/16 is the major network number and 172.16.0.0/24 is an all-zeros subnet. 172.16.255.255/16 and 172.16.255.255/24 are just as distinguishable.

By default the Cisco IOS rejects any attempt to configure an all-zeros subnet as an invalid address/mask combination even if a classless routing protocol is running. To override this default behavior, enter the global command ip subnet-zero.

A much greater benefit of having a subnet mask associated with each route is being able to use variable-length subnet masking (VLSM) and to summarize a group of major network addresses with a single aggregate address. Variable-length subnet masks, and address aggregation or supernetting, are examined with greater detail in Module 5, EIGRP.

| Interactive Media Activity Checkbox: Classless Routing Protocols Upon completion of this activity, the student will be able to identify the difference between classful and classless routing protocols.