3Com 3C840 manual Figure B-1Numbered WAN Interfaces

B-8APPENDIX B: IP ADDRESSING

exists when an IP address identifies the interface as belonging to a specific IP network or subnet (See Figure B-1).

Figure B–1Numbered WAN Interfaces

An unnumbered link exists when the IP address 0.0.0.0 is assigned and indicates the interface does not belong to a specific network (see Figure B-2).

Figure B-2Unnumbered WAN Interfaces

The major advantage of using unnumbered links is that you save scarce IP address space. Looking at Figure B-1, you can see that three IP networks are in use, 128.111.11.x (LAN A), 206.222.22.x (WAN), and 213.133.33.x (LAN B). The WAN network numbers are not needed when unnumbered is used. If the unnumbered scheme is used at only one end of the WAN link, a single IP address is saved. If it is used at both ends of the WAN link, an entire IP subnet will be saved.

At first glance it seems that unnumbered addressing would cause problems for routers. One expects each interface to be assigned an IP address. When the router sends router-generated packets (such as RIP packets or PINGs) the source IP address in the packet is typically the address of the interface the packet goes out. There could also be a concern about how to route packets to an unnumbered interface.

There are a few rules which take care of the problems. The Ethernet interface cannot be unnumbered. The router adopts a “router id,” which for the OfficeConnect Remote 840 is the IP address assigned to the