Summary of Module 1

Summary

This page summarizes the topics discussed in this module.

Variable-Length Subnet Masks (VLSM), often referred to as "subnetting a subnet", is used to maximize addressing efficiency. It is a feature that allows a single autonomous system to have networks with different subnet masks. The network administrator is able to use a long mask on networks with few hosts, and a short mask on subnets with many hosts.  

It is important to design an addressing scheme that allows for growth and does not involve wasting addresses. To apply VLSM to the addressing problem, large subnets are created for addressing LANs. Very small subnets are created for WAN links and other special cases.

VLSM helps to manage IP addresses. VLSM allows for the setting of a subnet mask that suits the link or the segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.

Addresses are assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits. There are specific rules for a router. It must know in detail the subnet numbers attached to it and it does not need to tell other routers about each individual subnet if the router can send an aggregate route for a set of routers. A router using aggregate routes would have fewer entries in its routing tables.

If VLSM is the scheme chosen, it must then be calculated and configured correctly.

RIP v1 is considered an interior gateway protocol that is classful. RIP v1 is a distance vector protocol that broadcasts its entire routing table to each neighbor router at predetermined intervals. The default interval is 30 seconds. RIP uses hop count as a metric, with 15 as the maximum number of hops.

To enable a dynamic routing protocol, select a routing protocol, such as RIP v2, assign the IP network numbers without specifying the subnet values, and then assign the network or subnet addresses and the appropriate subnet mask to the interfaces. In RIP v2, the router command starts the routing process. The network command causes the implementation of three functions. The routing updates are multicast out an interface, the routing updates are processed if they enter that same interface, and the subnet that is directly connected to that interface is advertised. The version 2 command enables RIP v2.

The show ip protocols command displays values about routing protocols and routing protocol timer information associated with the router. Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging.