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The way subnets work, wouldn't connecting two router interfaces together require their own subnet between them. Unless that subnet mask has 31 bits, wouldn't that was adress space. I'm asking because I often seen that done in networking books. How can this be done without wasting IP Addresses? They usually draw this when explaining subnetting. They have a central router connected to several other each one supposed to be creating their own subnet. Is this really how subnettimg is done?

Example

<-------[Router 1]-----Wasteful Subnet-------[Router 2]------>
             |
             |
             |
            \/
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The second router could perhaps be more secure (or, it is a textbook, they do a lot of odd things in those), or it could also be a bridge / something else –  ekaj Dec 2 '12 at 8:41
    
I have two routers in my setup. One is for static IP's and the other gives out DHCP IP's. It's the only solution I've found to stop my family's DHCP devices from stealing my servers static IP addresses! –  Ian Atkin Dec 2 '12 at 21:15

1 Answer 1

up vote 3 down vote accepted

It might, and if the two routers are controlled by different organizations, it might well be done this way. Otherwise, there are at least three ways you can avoid wasting IP addresses:

  1. The link can be bridged on one end or the other rather than routed.

  2. The link can be unnumbered.

  3. The link can use private IP addresses.

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What is a bridged connection and how can it solve this without using IP address space, also I thought you could only use private address space behind a NAT how would this work without one? –  rubixibuc Dec 2 '12 at 23:54
    
A bridged connection means the router acts more like a switch than a router and it can have more than one interface in the same broadcast domain. Private address space works perfectly well without NAT, the only issue is that devices assigned only private address space can't communicate over the Internet -- but if you have no devices assigned only private address space (or they have at least one private address behind a router that does support NAT), that's not an issue. If the link's addresses are used only for exchanging packets between the two routers, there's no issue. –  David Schwartz Dec 2 '12 at 23:57
    
Could you use private address space to link to routers that form part of a backbone, basically can it be dome anywhere throughout the Internet –  rubixibuc Dec 3 '12 at 0:47
    
@rubixibuc: Yes. The only issue is ICMP errors that might get the source IP address of one end of the link. Some people just don't bother. Some routers have an option to force the source IP address of ICMP responses to be the router's public IP address. –  David Schwartz Dec 3 '12 at 1:31

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