Information Technology Reference
In-Depth Information
Determining the Network Portion of an IP Address
Given an address and mask, you can determine the classful network, the subnetwork, and
the subnetwork's broadcast number. You do so with a logical AND operation between the
IP address and subnet mask. You obtain the broadcast address by taking the subnet num-
ber and making the host portion all 1s. Table 8-18 shows the logical AND operation. No-
tice that the AND operation is similar to multiplying bit 1 and bit 2; if any 0 is present, the
result is 0.
Ta b l e 8 -1 8
AND Logical Operation
Bit 1
Bit 2
AND
0
0
0
0
1
0
1
0
0
1
1
1
As an example, take the IP address 150.85.1.70 with a subnet mask of 255.255.255.224,
as shown in Table 8-19. Notice the 3 bold bits in the subnet mask. These bits extend the
default Class C prefix (/24) 3 bits to a mask of /27. As shown in Table 8-19, you perform
an AND operation of the IP address with the subnet mask to obtain the subnetwork. You
obtain the broadcast number by making all the host bits 1. As shown in bold, the subnet
mask reaches 3 bits in the fourth octet. The subnetwork is identified by the five right-
most 0s in the fourth octet, and the broadcast is identified by all 1s in the 5 rightmost
bits.
Ta b l e 8 -1 9
Subnetwork of IP Address 150.85.1.70
Binary First, Second, and Third
Octets
Binary Fourth
Octet
Dotted-Decimal
IP
IP address
10010110 01010101 00000001
010
00110
150.85.1.70
Subnet mask
11111111 11111111 11111111
111
00000
255.255.255.224
Subnetwork
10010110 01010101 00000001
010
00000
150.85.1.64
Major network portion
Subnet
Host
Broadcast
address
10010110 01010101 00000001
010
11111
150.85.1.95
