Editing IPv4 (section)

===Unnumbered interface===
An unnumbered [[Point-to-point (telecommunications)|point-to-point]] (PtP) link, also called a transit link, is a link that does not have an IP network or subnet number associated with it, but still has an IP address. First introduced in 1993,<ref>{{cite journal|journal=Internet Engineering Task Force|last1=Almquist|first1=Philip|last2=Kastenholz|first2=Frank|date=December 1993|title=Towards Requirements for IP Routers|url=https://datatracker.ietf.org/doc/draft-ietf-rreq-iprouters-require/00}}</ref>{{Ref RFC|1716}}{{Ref RFC|1812}}<ref>{{Cite web|title=Understanding and Configuring the ip unnumbered Command|url=https://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/13786-20.html|access-date=2021-11-25|website=Cisco|language=en}}</ref> Phil Karn from Qualcomm is credited as the original designer.

The purpose of a transit link is to [[Router (computing)|route]] [[datagram]]s. They are used to free IP addresses from a scarce IP address space or to reduce the management of assigning IP and configuration of interfaces. Previously, every link needed to dedicate a {{IPaddr||31}} or {{IPaddr||30}} subnet using 2 or 4 IP addresses per point-to-point link. When a link is unnumbered, a ''router-id'' is used, a single IP address borrowed from a defined (normally a [[loopback]]) interface. The same ''router-id'' can be used on multiple interfaces.

One of the disadvantages of unnumbered interfaces is that it is harder to do remote testing and management.