7.13: Unnumbered Interfaces

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    From LibreTexts, under CC BY-NC-ND 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    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 200.0.0.0/24 and 201.1.1.0/24 respectively. The endpoint interfaces of L, both labeled link0, are unnumbered.

    unnumbered.svg

    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, 200.0.0.1; 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 201.1.1.1 but has no interface that is assigned an IP address on the destination subnet 201.1.1.0/24. 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

    The Linux ip route command illustrated here was tested on a virtual point-to-point link created with ssh and pppd; the link interface name was in fact ppp0. While the command appeared to work as advertised, it was only possible to create the link if endpoint IP addresses were assigned at the time of creation; these were then removed with ip route del and then re-assigned with the command shown here.

    ip route add 201.1.1.1 dev link0

    Because L is a point-to-point link, there is no destination LAN address and thus no ARP query.

    This page titled 7.13: Unnumbered Interfaces was last modified on Mon, 18 May 2020 08:24:33 GMT and is shared under a CC BY-NC-ND 4.0 license and was authored, remixed, and/or curated by Peter Lars Dordal.