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Unformatted text preview: address) and our subnet mask is 255.255.255.0
(8 bits for the subnet ID and 8 bits for the host ID).
q q q If a destination IP address is 126.96.36.199, we know that the class B network IDs are the
same (140.252), but the subnet IDs are different (1 and 4). Figure 3.8 shows how this
comparison of two IP addresses is done, using the subnet mask.
If the destination IP address is 188.8.131.52, the class B network IDs are the same
(140.252), and the subnet IDs are the same (1). The host IDs, however, are different.
If the destination IP address is 184.108.40.206 (a class C address), the network IDs are
different. No further comparisons can be made against this address. Figure 3.8 Comparison of two class B addresses using a subnet mask. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (12 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol The IP routing function makes comparisons like this all the time, given two IP addresses and a
subnet mask. 3.6 Special Case IP Addresses
Having described subnetting we now show the seven special case IP addresses in Figure 3.9. In this
figure, 0 means a field of all zero bits, -1 means a field of all one bits, and netid, subnetid, and
hostid mean the corresponding field that is neither all zero bits nor all one bits. A blank subnet ID
column means the address is not subnetted.
net ID subnet
ID Can appear as
host ID Description source? destination?
this host on this net (see restrictions below)
specified host on this net (see restrictions
never 127 anything OK OK loopback address (Section 2.7) -1
OK limited broadcast (never forwarded)
net-directed broadcast to netid
subnet-directed broadcast to netid, subnetid
all-subnets-directed broadcast to netid subnetid
1 Figure 3.9 Special case IP addresses.
We have divided this table into three sections. The first two entries are special case source
addresses, the next one is the special loopback address,...
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