3Com 3C840 - Manual (Page 180)

B-8 A PPENDIX B: IP A DDRESSING
exists when an IP address identifies the interface as belonging to a
specific IP network or subnet (See Figure B-1).
Figure B–1 Numbered 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-2 Unnumbered 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
840ug.book Page 8 Friday, July 7, 2000 2:23 PM