How would a router with /32 WAN subnet mask communicate with the rest of the Internet?

  • I'm studying networking and one of the most basic things I've been told countless times is that computers can't communicate outside of their own subnet.

    On the other hand, by looking at some ADSL setups, I've seen that in case where a fixed IP address is assigned to the user's WAN interface, a /32 subnet mask is used for the WAN interface and the default gateway is of course outside of the subnet.

    So how would the host with /32 subnet communicate with outside network?

    Here's a sort of a diagram of what I have:

    Network Diagram

    The DSL modem is set up to act as a bridge so Router0 does authentication and sets up connection via PPPoE to the ISP. The Router0 gets an IP address on its WAN interface (Fa0/0 on the diagram) which is from a /32 subnet using IPCP. It only receives a single IP address on the WAN interface and IP aliasing isn't being used here.

    I understand what goes on from the Router0 to the LAN. What I don't understand is how does the Router0 communicate with ISP.

  • Ricky

    Ricky Correct answer

    9 years ago

    DSL tends to use PPPoE, which means the link is point-to-point. A /32 is perfectly valid here. In fact, no address at all is still valid -- one would need routable addresses beyond the PPP link, 'tho. In this case, it works because there's only one possible destination on the link. Every packet put on that link will be processed by the other end, and v.v.

    blue-gw#show int di1
     Dialer1 is up, line protocol is up (spoofing)
     Description: Bellsouth.net DSL
     Internet address is 74.167.x.x/32
    
    Routing:
     Gateway of last resort is 72.157.24.5 to network 0.0.0.0
     ...
          72.0.0.0/32 is subnetted, 1 subnets
     C       72.157.24.5 is directly connected, Dialer1
          74.0.0.0/32 is subnetted, 1 subnets
     C       74.167.x.x is directly connected, Dialer1
    

    Could you please explain a bit more how the /32 is fine on point-to-point link? I understand that on a point-to-point link, the data has nowhere to go except to the other side, but in such case, I'd expect to see the interface set up as the exit point in the routing table and not the IP address of the default gateway. If the explanation is too complicated to fit an answer, some keywords for Google would be good too.

    PPP reports local and remote side addresses, so the interface will be set with the local (usually /32) and the default gateway will be the remote. A route to the remote via the PPP interface will be auto-generated. (see edit)

License under CC-BY-SA with attribution


Content dated before 7/24/2021 11:53 AM

Tags used