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IP addressing scheme-subnets and mask

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by ts1026

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I am about to upgrade my WAN with Cisco routers and a Cisco Pix firewall. I have vendors coming in to install and configure. We have 3 locations and everything will be accessed at headquarters (including the internet). The subnets they are givingme are 10.16.19.x, 10.16.20.x, and 10.16.21.x. They are giving me a mask of 255.255.255.0. My ISP has to reconfigure my cable router for the new IP addresses. They said they have to use a mask of 255.255.0.0 for the 3 subnets. Is this going to work?

4 replies

by timwalsh

Yes, it should.

The subnet mask is used to determine what part of the IP address determines the host in a given network. Each of your subnets uses only the last octet to determine the host, hence the 255.255.255.0 subnet mask. However, since your overall network consists of multiple Class C subnets, your corporate network is essentially a Class B network (10.16.x.x), and thus needs a Class B subnet mask (255.255.0.0).

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