7.13: Unnumbered Interfaces
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We mentioned in 1.10 IP - Internet Protocol and 7.2 Interfaces that some devices allow the use of point-to-point IP links without assigning IP addresses to the interfaces at the ends of the link. Such IP interfaces are referred to as unnumbered; they generally make sense only on routers. It is a firm requirement that the node (ie router) at each endpoint of such a link has at least one other interface that does have an IP address; otherwise, the node in question would be anonymous, and could not participate in the router-to-router protocols of 9 Routing-Update Algorithms.
The diagram below shows a link L joining routers R1 and R2, which are connected to subnets 18.104.22.168/24 and 22.214.171.124/24 respectively. The endpoint interfaces of L, both labeled
link0, are unnumbered.
The endpoints of L could always be assigned private IPv4 addresses (7.3 Special Addresses), such as 10.0.0.1 and 10.0.0.2. To do this we would need to create a subnet; because the host bits cannot be all 0’s or all 1’s, the minimum subnet size is four (eg 10.0.0.0/30). Furthermore, the routing protocols to be introduced in 9 Routing-Update Algorithms will distribute information about the subnet throughout the organization or “routing domain”, meaning care must be taken to ensure that each link’s subnet is unique. Use of unnumbered links avoids this.
If R1 were to originate a packet to be sent to (or forwarded via) R2, the standard strategy is for it to treat its
link0 interface as if it shared the IP address of its Ethernet interface
eth0, that is, 126.96.36.199; R2 would do likewise. This still leaves R1 and R2 violating the IP local-delivery rule of 7.5 The Classless IP Delivery Algorithm; R1 is expected to deliver packets via local delivery to 188.8.131.52 but has no interface that is assigned an IP address on the destination subnet 184.108.40.206/24. The necessary dispensation, however, is granted by RFC 1812 [https://tools.ietf.org/html/rfc1812.html]. All that is necessary by way of configuration is that R1 be told R2 is a directly connected neighbor reachable via its
link0 interface. On Linux systems this might be done with the
ip route command on R1 as follows:
ip route add 220.127.116.11 dev link0
Because L is a point-to-point link, there is no destination LAN address and thus no ARP query.