258 Chapter 5: Network Protocols
Given an IP Address and Mask, What Are the Assignable IP Addresses in That Network/
Subnet?
CCNAs deal with the question, "What IP addresses are in this subnet?" on a regular basis. This
section describes how to answer this question if you know how to derive the subnet and
broadcast addresses. Simply put, the valid IP addresses that are available for assignment in a
subnet are those numerically between the subnet number and the broadcast address.
There is little else to be considered. Certainly, no decimal or binary algorithm needs to be
considered for such a simple concept, right? Table 5-19 shows some familiar IP addresses and
the corresponding IP addresses in the same subnet.
The ranges seem obvious. However, look at the 172.100.200.0 subnet. How many assignable
IP addresses are in that subnet, and what are those addresses? Is 172.100.200.255 valid? What
about 172.100.201.0? In fact, both are valid IP addresses when using 255.255.254.0 as the
Broadcast address (boring octets to the left) (Step 5)
172.100.____.____
Broadcast address (boring octets to the right) (Step 6)
172.100.____.255
Broadcast address (202 is the closest multiple of 2 and is greater than 201;
202 1 = 201) (Step 7)
172.100.201.255
Address (Step 1)
17.9.44.70
Mask (Step 2)
255.255.255.192
Interesting octet (Step 3)
4
Multiplier (Step 4)
256 192 = 64
Broadcast address (boring octets to the left) (Step 5)
17.9.44.____
Broadcast address (boring octets to the right) (Step 6)
17.9.44.____
Broadcast address (128 is the closest multiple of 64 and is greater than 70;
128 1 = 127) (Step 7)
17.9.44.127
Table 5-19
Assignable Addresses
Subnet Number
Subnet Mask
Broadcast
Address
Range of Assignable
Addresses
130.4.100.0
255.255.252.0
130.4.103.255
130.4.100.1 to 130.4.103.254
199.1.1.0
255.255.255.224
199.1.1.31
199.1.1.1 to 199.1.1.30
172.100.200.0
255.255.254.0
172.100.201.255
172.100.200.1 to
172.100.201.254
17.9.44.64
255.255.255.192
17.9.44.127
17.9.44.65 to 17.9.44.126
ch05.fm Page 258 Monday, March 20, 2000 5:06 PM