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What is the IP address(es) of the router in my picture? http://oi60.tinypic.com/x0quqv.jpg

In my picture, you can see that I have question marks next to the router's interfaces, because I'm not sure if those are right. Can someone help me out here?

I drew this picture after reading this: https://serverfault.com/a/226445/228651. It is a diagram of how I think the router and subnets described in that link would be laid out. Basically, there is a subnet with network ID 192.168.40.0 and a subnet mask of 255.255.255.0 (/24). That subnet is divided up by Router A which handles traffic to and from 3 of its own sub-subnets.

Perhaps I'm missing something fundamental, but I'm of the opinion that the router does not even necessarily need an IP address(es). Other routers/network switches that connect to this router just need to know the IP address of the final destination device, and use its own routing table to decide which of its own interfaces to send packets out of (the IP address of the intermediary router at the other end of that interface would be unnecessary information).

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Niko Bellic
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  • Yes, you're missing something fundamental: https://en.wikipedia.org/wiki/IP_forwarding_algorithm – sciurus Jul 12 '14 at 01:36
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    The router could have any IP address within the network of each interface (excluding the broadcast address and the network address). – DerfK Jul 12 '14 at 02:03

2 Answers2

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On most of the networks I work with the router address is the first routable address (1, 129, and 193) in the subnet. This is by tradition, and not a requirement. Another option is to use the highest routeable address (126, 190, and 254) for the router. The lowest address in the each subnet (0, 128, and 192) is the network address and is not assigned. The highest address in each subnet (127, 191, and 255) is the broadcast address and should be received by all computers in the subnet. To allow for these three addresses, the smallest subnet has four addresses, allowing for one computer in the subnet.

I see a few problems with the diagram:

  • The Internet address will be on a different network than the network behind the router. The address in the diagram is wrong. If the router and its upstream router support point-to-point routing, it might be* possible to use one of the routers address on the upstream link.
  • The networks on the /26 subnets have the maximium number of computers, but highest numbered computer should be 60 higher than the lowest. You have added 61. If you use the lowest routable address for the router they have the correct address and you should add 1 to the IP addresses for computer 1, and 2.
  • The network on the /25 subnet has room for only 125 (128 - 3) computers. I would rename the computer 126 to computer 125 and increase the addresses of computer 1 and 2 as above.
BillThor
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  • Thank you BillThor! Here is a picture which includes your fixes: http://oi61.tinypic.com/2rxd79i.jpg. I put a random, different IP address on the internet side of the router. You're answer was really, really helpful! – Niko Bellic Jul 12 '14 at 02:44
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Those addresses are network addresses, which identify the specific network, and cannot be assigned to a host, which is what a router is. A router's interface must have a host address in order to identify that it is a member of that network and to know what interface to route traffic to.

Davidw
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