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Unformatted text preview: for routing decisions. 3.4 Subnet Addressing
All hosts are now required to support subnet addressing (RFC 950 [Mogul and Postel 1985]). file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (9 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol Instead of considering an IP address as just a network ID and host ID, the host ID portion is divided
into a subnet ID and a host ID.
This makes sense because class A and class B addresses have too many bits allocated for the host
ID: 224-2 and 216-2, respectively. People don't attach that many hosts to a single network. (Figure
1.5 shows the format of the different classes of IP addresses.) We subtract 2 in these expressions
because host IDs of all zero bits or all one bits are invalid.
After obtaining an IP network ID of a certain class from the InterNIC, it is up to the local system
administrator whether to subnet or not, and if so, how many bits to allocate to the subnet ID and
host ID. For example, the internet used in this text has a class B network address (140.252) and of
the remaining 16 bits, 8 are for the subnet ID and 8 for the host ID. This is shown in Figure 3.5. Class B 16 bits
netid = 140.252
Figure 3.5 Subnetting a class B address. 8 bits
hostid This division allows 254 subnets, with 254 hosts per subnet.
Many administrators use the natural 8-bit boundary in the 16 bits of a class B host ID as the subnet
boundary. This makes it easier to determine the subnet ID from a dotted-decimal number, but there
is no requirement that the subnet boundary for a class A or class B address be on a byte boundary.
Most examples of subnetting describe it using a class B address. Subnetting is also allowed for a
class C address, but there are fewer bits to work with. Subnetting is rarely shown with a class A
address because there are so few class A addresses. (Most class A addresses are, however,
Subnetting hides the details of internal network organization (within a company or campus) t...
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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