Unformatted text preview: send the datagram to R55, who then sends it to R57. 3.5 Subnet Mask
Part of the configuration of any host that takes place at bootstrap time is the specification of the
host's IP address. Most systems have this stored in a disk file that's read at bootstrap time, and we'll
see in Chapter 5 how a diskless system can also find out its IP address when it's bootstrapped.
In addition to the IP address, a host also needs to know how many bits are to be used for the subnet
ID and how many bits are for the host ID. This is also specified at bootstrap time using a subnet
mask. This mask is a 32-bit value containing one bits for the network ID and subnet ID, and zero
bits for the host ID. Figure 3.7 shows the formation of the subnet mask for two different partitions
of a class B address. The top example is the partitioning used at noao.edu, shown in Figure 3.5, file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (11 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol where the subnet ID and host ID are both 8 bits wide. The lower example shows a class B address
partitioned for a 10-bit subnet ID and a 6-bit host ID. Figure 3.7 Example subnet mask for two different class B subnet arrangements.
Although IP addresses are normally written in dotted-decimal notation, subnet masks are often
written in hexadecimal, especially if the boundary is not a byte boundary, since the subnet mask is
a bit mask.
Given its own IP address and its subnet mask, a host can determine if an IP datagram is destined for
(1) a host on its own subnet, (2) a host on a different subnet on its own network, or (3) a host on a
different network. Knowing your own IP address tells you whether you have a class A, B, or C
address (from the high-order bits), which tells you where the boundary is between the network ID
and the subnet ID. The subnet mask then tells you where the boundary is between the subnet ID
and the host ID.
Assume our host address is 22.214.171.124 (a class B...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12