I have a Cisco router with four serial interfaces, two dedicated p2p and two frame sub interfaces. Each interface address is part of the 192.168.130 subnet. The hosts are numbered 13,33,26,22 with a 30-bit mask, respectively.

Everything works OK, but plugging the subnet and mask into a calculator yields invalid host for 1 and 2.

Everything is working, and has been for some time, but I need to understand a little better just why? The SNM is used by the router to talk to the interface on the other end of the circuit, why isn't it choking on the host part of the address?

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I think you are putting the following subnet and mask values to calculate the host IP addresses
Subnet 192.168.130.0
Mask 255.255.255.252
In your case you actually have four subnets as follows:
Subnet for host 13 is 192.168.130.12 mask 255.255.255.252
Subnet for host 22 is 192.168.130.20 mask 255.255.255.252
Subnet for host 26 is 192.168.130.24 mask 255.255.255.252
Subnet for host 33 is 192.168.130.32 mask 255.255.255.252

If you use these values I think you should get the correct values for the hosts. I am answering this question on theoretical basis, because you have not provided any information about the type of subnet calculator you are using. There might also be a bug in your calculator. I hope this answers your question.

For more information, see our guide to IP addressing and subnetting fundamentals.

This was first published in January 2004

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