TCP IP Illustrated

The subnet mask then tells you where the boundary is

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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 140.252.4.5, 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 140.252.1.22, 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 192.43.235.6 (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. IP address 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 below) 0 0 0 hostid OK OK never never 127 anything OK OK loopback address (Section 2.7) -1 netid netid netid -1 -1 -1 -1 never never never never OK OK OK 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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