IronPort Systems ES-1528 User Manual download pdf (Page 155)

Appendix B IP Addresses and Subnetting

ES-1528 User’s Guide

155

Figure 97 Subnetting Example: After Subnetting

In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of 2

– 2 or 126

possible hosts (a host ID of all zeroes is the subnet’s address itself, all ones is the subnet’s

broadcast address).

192.168.1.0 with mask 255.255.255.128 is subnet A itself, and 192.168.1.127 with mask

255.255.255.128 is its broadcast address. Therefore, the lowest IP address that can be assigned

to an actual host for subnet A is 192.168.1.1 and the highest is 192.168.1.126.

Similarly, the host ID range for subnet B is 192.168.1.129 to 192.168.1.254.

Example: Four Subnets

The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two

subnets. Similarly, to divide a 24-bit address into four subnets, you need to “borrow” two host

ID bits to give four possible combinations (00, 01, 10 and 11). The subnet mask is 26 bits

(11111111.11111111.11111111.11000000) or 255.255.255.192.

Each subnet contains 6 host ID bits, giving 2

- 2 or 62 hosts for each subnet (a host ID of all

zeroes is the subnet itself, all ones is the subnet’s broadcast address).

Table 78 Subnet 1

IP/SUBNET MASK NETWORK NUMBER

LAST OCTET BIT

VALUE

IP Address (Decimal) 192.168.1. 0

IP Address (Binary) 11000000.10101000.00000001. 00000000

Subnet Mask (Binary) 11111111.11111111.11111111. 11000000

Subnet Address:

192.168.1.0

Lowest Host ID: 192.168.1.1

Broadcast Address:

192.168.1.63

Highest Host ID: 192.168.1.62

1 2 ... 150 151 152 153 154 155 156 157 158 159 160 ... 169 170

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IronPort Systems ES-1528 User Manual Page 155