About Subnet Masks; About Slash Notation; About Entering Ip Addresses - Watchguard Firebox X15 User Manual

Introduction to Network Security

About subnet masks

Because of security and performance considerations, networks are often divided into smaller portions called
subnets. All devices in a subnet have similar IP addresses. For example, all devices that have IP addresses
whose first three octets are 50.50.50 would belong to the same subnet.
A network IP address's subnet mask, or netmask, is a string of bits that mask sections of the IP address to show
how many addresses are available and how many are already in use. For example, a large network subnet
mask might look like this: 255.255.0.0. Each zero shows that a range of IP addresses from 1 to 255 is available.
Each decimal place of 255 represents an IP address range that is already in use. In a network with a subnet
mask of 255.255.0.0, there are 65,025 IP addresses available. A smaller network subnet mask is 255.255.255.0.
Only 254 IP addresses are available.

About slash notation

The Firebox uses slash notation for many purposes, including policy configuration. Slash notation is a compact
way to show the subnet mask for a network. To write slash notation for a subnet mask:
1. First, find the binary representation of the subnet mask.
For example, the binary representation of 255.255.255.0 is 11111111.11111111.11111111.00000000.
2. Count each 1 in the subnet mask.
This example has twenty-four (24) of the numeral 1.
3. Add the number from step two to the IP address, separated by a forward slash (/).
The IP address 192.168.42.23/24 is equivalent to an IP address of 192.168.42.23 with a netmask of
255.255.255.0.
This table shows common network masks and their equivalents in slash notation.
Network mask
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252

About entering IP addresses

When you type IP addresses in the Quick Setup Wizard or dialog boxes in Firebox management software, type
the digits and periods in the correct sequence. Do not use the TAB key, arrow key, spacebar, or mouse to put
your cursor after the periods.
For example, if you type the IP address 172.16.1.10, do not type a space after you type 16. Do not try to put
your cursor after the subsequent period to type 1. Type a period directly after 16, and then type 1.10. Press the
slash (/) key to move to the netmask.
4
Slash equivalent
/8
/16
/24
/25
/26
/27
/28
/29
/30
Firebox X Edge e-Series