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IP Addressing and Subnetting 245
3
Given an IP address and mask, what are the assignable IP addresses in that network/
subnet?
4
Given a network number and a static subnet mask, what are the valid subnet numbers?
5
Given a network number and a static subnet mask, how many hosts per subnet, and how
many subnets?
This section provides both a decimal and a binary algorithm for each perspective on IP
addressing. The decimal processes will help you find the answers more quickly; the binary
algorithms will help you more fully understand IP addressing.
Given an IP Address and Mask, What Is the Network/Subnet Number?
Both people and computers need to think about the question, "Which network is a particular
address a member of?" Humans care because it is useful in troubleshooting, planning, and
address assignment; computers need to know because the answer is a vital part of routing.
Decimal Algorithm for Deriving the Network Number, No Subnetting in Use
When no subnetting is in use, the decimal algorithm is as follows:
Step 1
Write down the IP address in decimal.
Step 2
Copy below the IP address either the first one, two, or three dotted
decimal numbers of the address, based on whether the address is
a Class A, B, or C address, respectively.
Step 3
For the remaining dotted decimal numbers, record decimal
value 0.
Table 5-14 shows some examples for deriving the network number (no subnetting in use) based
on the steps in the preceding list.
Table 5-14
Example Dissections of IP Addresses, No Subnetting
IP Address (Step 1)
Network Part (Step 2)
Network Number (Step 3)
8.1.4.5
8
8.0.0.0
130.4.100.1
130.4
130.4.0.0
199.1.1.4
199.1.1
199.1.1.0
172.100.2.2
172.100
172.100.0.0
ch05.fm Page 245 Monday, March 20, 2000 5:06 PM