Setup guide

Appendix E – IP Address Range and
Subnets
IP Address and Subnet Masks
It is possible to set a range of IP address within the Firewall that will have access to the unit. The
following details how the address range and subsequent subnet is calculated and can be used in
conjunction with the Configuration section of this manual.
An IP address is a 32 bit address that is read by the network devices (switches, hubs, routers) in a
binary format, however to make life simpler for Network Administrator, IP addresses are displayed in
a decimal format.
The same applies to subnet masks, these to are 32 bit addresses and are identified by the network
devices in binary format, but for written in a decimal format.
The 32 bits are grouped in to four 8 bits (an Octet) to give us the IP address format we are used to:
Binary Format Decimal Equivalent
11000000.10101000.00000011.00000001 192.168.3.1
11111111.11111111.11111111.00000000 255.255.255.0
11.0002
The binary format of the IP address uses 1’s and 0’s and within an octet it is possible to identify 256
decimal numbers from 0 to 255.
11.0003
128 64 32 16 8 4 2 1 Decimal
1 1 1 1 1 1 1 1 255
0 0 0 0 0 0 0 0 0
11.0004
An IP address along with its subnet mask is made up of two parts; Network ID and Host ID. If we
use our example IP address, 192.168.3.1 we can see the network ID and the host ID;
IP Address Network ID Host ID
192.168.3.1 192.168.3 .1
255.255.255.0 255.255.255 .0
11.0005
Wherever the subnet gives a value of 255 (all 1’s) it ‘masks’ out the IP address octet and therefore
represent the Network part of the overall IP address. Hence the reason the above example takes the
first three octets as the network ID and the last octet as the host ID.
11.1001
Class A
The Class A network allocates the first octet to the Network ID and the remaining three octets are the
Host ID’s. There is also an address range that has been defined for a Class A network. As we use the
first octet as the Network ID, we can see from the subnet mask that a Class A subnet ‘masks’ out the
network portion of the address.
Class A Address Range Subnet Mask Alternative Format
0.x.x.x 255.0.0.0 0.x.x.x/8
126.x.x.x 255.0.0.0 126.x.x.x/8
11.1002
Classes of Networks
There are three ‘available’ classes of networks. There other classes of networks that are reserved and
therefore are not ‘available’ to the general public.
11.1003