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I have an address zone (let's say and I want to subnet it optimally using VLSM. I have a number of VLANs, each with a minimum number of needed addresses, and a couple of router-to-router connections.

How many addresses are needed for a router-to-router connection (that is considered as a subnetwork)?

Is it four? One for each router, the broadcast address and the network address?

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up vote 3 down vote accepted

That depends on whether your routers support RFC 3021 addressing or not. Typically you'd use a /30 subnet for a point-to-point, but RFC 3021 allows for /31's.


If you use /30's then you have 2 bits of host ID. If you use /31's then you have 1 bit of host ID. Do the math... >smile<

Smells like homework, BTW.

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Well it is part of a larger homework. I have a packet tracer configuration. The first step is subnetting my adress zone for the given configuration. I wasn't sure how to consider the subnetwork between two routers.. – biggdman Nov 23 '11 at 22:20
@biggdman: Forgive me if I'm being a little evasive in my answer. You can find a lot more detail at: Your instructor needs to tell you if the routers support RFC 3021 or not. Modern gear would. – Evan Anderson Nov 23 '11 at 22:21
Thanks for the answer. Also a side problem to this matter I don't really understand: if I have 4 Routers connected through switches, would 6 adresses be enough for such a subnetwork( 1 for each router and 1 for the subnetwork adress and 1 for the broadcast adress), and a total of 3 bits of host(/29)? – biggdman Nov 23 '11 at 22:45
@Biggdman: Answer your own question-- you just laid out all the facts. Are there enough bits of host ID in a /29 to handle 4 hosts, the subnet address, and the broadcast address? – Evan Anderson Nov 23 '11 at 22:52

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