Discontiguous subnets

Unanswered Question
Mar 20th, 2007


Can you help in getting correct definition and notes for topic "Discontigueous subnets" since iam confuse on that

Thanx in advance


I have this problem too.
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Richard Burts Tue, 03/20/2007 - 05:48


In traditional classful IP addressing it was assumed that subnets would be contiguous. What that means is basically that to get from one subnet of some major network to any other subnet of that network, you would go through only subnets of that network. For example if you were in subnet and you were going to you might go through That would be contiguous.

Discontiguous subnets means that to go from one subnet of a network to another subnet of that network you must go through subnets of a different network. For example if you are in subnet and to get to you must go through then that would be discontiguous.

In traditional classful routing protocols (such as RIP and IGRP) discontiguous subnets are a problem. In more modern classless routing protocols such as OSPF discontiguous subnets are less of a problem.



carl_townshend Tue, 03/20/2007 - 06:32

why would they be a problem ? im sure contigeous would be an issue as they are classful and so if you had say 2 routers advertising and, this would not work as the traffic would go to both as rip would advertise the subnet !

jcorrale Tue, 03/20/2007 - 05:55


Here are useful documents about Discontiguos Subnets:



"IP subnet design traditionally has not allowed discontiguous networks. A contiguous network is a single Class A, B, or C network for which all routes to subnets of that network pass through only other subnets of that same single network. Discontiguous networks refer to the concept that, in a single Class A, B, or C network, there is at least one case in which the only routes to one subnet pass through subnets of a different network. An easy analogy for residents in the United States is the familiar term contiguous 48, referring to the 48 states besides Alaska and Hawaii. To drive to Alaska from the contiguous 48, for example, you must drive through another country (Canada, for the geographically impaired!), so Alaska is not contiguous with the 48 states?in other words, it is discontiguous."

Hope it helps!




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