Unformatted text preview: and the final four are the broadcast
"The first two entries in the table, with a network ID of 0, can only appear as the source address as
part of an initialization procedure when a host is determining its own IP address, for example, when
the BOOTP protocol is being used (Chapter 16). In Section 12.2 we'll examine the four types of
broadcast addresses in more detail. 3.7 A Subnet Example
This example shows the subnet used in the text, and how two different subnet masks are used.
Figure 3.10 shows the arrangement. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (13 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol Figure 3.10 Arrangement of hosts and networks for author's subnet.
If you compare this figure with the one on the inside front cover, you'll notice that we've omitted
the detail that the connection from the router sun to the top Ethernet in Figure 3.10 is really a dialup
SLIP connection. This detail doesn't affect our description of subnetting in this section. We'll return
to this detail in Section 4.6 when we describe proxy ARP.
The problem is that we have two separate networks within subnet 13: an Ethernet and a point-topoint link (the hardwired SLIP link). (Point-to-point links always cause problems since each end
normally requires an IP address.) There could be more hosts and networks in the future, but not
enough hosts across the different networks to justify using another subnet number. Our solution is
to extend the subnet ID from 8 to II bits, and decrease the host ID from 8 to 5 bits. This is called
variable-length subnets since most networks within the 140.252 network use an 8-bit subnet mask
while our network uses an 11-bit subnet mask.
RFC 1009 [Braden and Postel 1987] allows a subnetted network to use more than one subnet mask. The new Router
Requirements RFC [Almquist 1993] requires support for this.
The problem, however, is that not all routing protocols exchange the subnet mask along with the destin...
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