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I don't understand why there has to be a separate network address. Aren't the DSN submask and the network broadcast addresses enough to identify a network?

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Please be more specific Network address can mean a few different things... I assume you mean like 192.168.1.0? That just signifies all hosts on the 192.168.1.1/24 subnet and is used when setting up static routes. –  Kyle Apr 6 '12 at 18:02

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Networks need specific addresses to support routing. Routers are configured to send traffic to different interfaces depending on the network address. They usually do not care about the specific host unless they are a router on the same subnet as that host.

For example, you send traffic from your PC, to your home router, which forwards it to your ISP's next level router. That router is likely not directly connected to the destination of your traffic, so it will be forwarding it to another router that is closer. This is all based on the network part of the address and not the host.

Network addresses are also important in "route summarization" which allows routing tables to be shorter - very important when hundreds or thousands of routes are involved.

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In answer to your first paragraph, why not just use the address of the sending host as the first address in the network, or the address of the local router or switch? Having a separate address that you cannot send to simply to talk about a network, or describe a network seems a waste. The route summarization part, I get (kind of), but it seems to me that the address of the lowest level network should be the address of the lowest level router. –  Paul Wiklund Apr 6 '12 at 19:29
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Having a separate network address prevents confusion, i.e. 192.168.1.0 can only mean a network. There may be other historical reasons for this that I'm unfamiliar with. Also, Networks can have multiple routers connecting them. There may not be a single "lowest level router" in a network. "Local router" may in fact be "local routers" for some networks, and some networks may only consist of routers. –  ultrasawblade Apr 6 '12 at 21:22
    
I hadn't thought of it that way. Thank you. :) –  Paul Wiklund Apr 6 '12 at 21:24

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