BSDwiki/ Gather information using an IP address and subnet mask

Gather information using an IP address and subnet mask

Author: Cezary Morga cm@therek.net FreeBSD

Reviewer: name contact BSD flavour

Reviewer: Yannick Cadin yannick@diablotin.fr FreeBSD/OpenBSD

Concept

Given an IPv4 address and subnet mask, be able to determine the subnet address, broadcast address and the valid host addresses available on that subnet address.

Introduction

Configuring BSD to work in network environment requires proficiency in operating on IP addresses and subnet masks. To understand different subnet mask's notations used throughout this section please refer to Convert a subnet mask between dotted decimal, hexadecimal or CIDR notation.

Examples

Having a host with an IP address of 192.168.1.25/24 we can determine without any further calculations:

""subnet identifier: 192.168.1.0 ""broadcast address: 192.168.1.255 ""number of valid host addresses: 254

The subnet mask of /24 in CIDR notation stands for 255.255.255.0. The IPv4 uses a 32-bit IP addressing and network masks, which means that the above subnet mask have last eight bits set to zero (32 - 24 = 8), which also mean that we have 2^8 IP addresses available. Subtracting two addresses for subnet address (first address) and broadcast address (last address) we get number of valid hosts:

""2^8 - 2 = 256 - 2 = 254

As for the address of 192.168.1.25/28 the paremeters in question are:

""subnet identifier: 192.168.1.16 ""broadcast address: 192.168.1.31 ""number of valid host addresses: 14

This is quite more interesting example. Having a CIDR subnet mask of /28 (four bits set to zero)--which is 255.255.255.240 in dotted decimal notation--means that there are 16 addresses available, and only 14 of them are valid host addresses:

""2^4 - 2 = 16 - 2 = 14

For networks smaller than 256 IP addresses there's a simple way to determine number of available addresses using a 'neat trick' as described in Daryl's TCP/IP Primer (link at the end of this subject). To do so we can simply subtract the last number of the subnet mask from 256. For aformentioned subnet mask of 255.255.255.240 we'll have 256 - 240 = 16 addresses. Now, dividing the result into 256 we can determine the number of subnets (256 / 16 = 16), which gives us a 16 subnets of 16 addresses each. The scope of the first one is 192.168.1.0 - 192.168.1.15, the second one 192.168.1.16 - 192.168.1.31 and so forth. Our IP address of 192.168.1.25 is located within the second one.

To make it all more confusing let's try to determine subnet's parameters having a 192.168.1.25/22 IP address:

""subnet identifier: 192.168.0.0 ""broadcast address: 192.168.3.255 ""number of valid host addresses: 1022

The CIDR subnet mask of /22 gives us 10 bits defining the hosts addresses.

""2^10 - 2 = 1024 - 2 = 1022

This gives us networks with a scope of IP addresses: 192.168.0.0 - 192.168.3.255, 192.168.4.0 - 192.168.7.255, etc.

In closing, the partial reference table on IPv4 subnets:

"" CIDR | Netmask | Addresses ""-----+-----------------+----------- "" /18 | 255.255.192.0 | 16384 "" /19 | 255.255.224.0 | 8192 "" /20 | 255.255.240.0 | 4096 "" /21 | 255.255.248.0 | 2048 "" /22 | 255.255.252.0 | 1024 "" /23 | 255.255.254.0 | 512 "" /24 | 255.255.255.0 | 256 "" /25 | 255.255.255.128 | 128 "" /26 | 255.255.255.192 | 64 "" /27 | 255.255.255.224 | 32 "" /28 | 255.255.255.240 | 16 "" /29 | 255.255.255.248 | 8 "" /30 | 255.255.255.252 | 4 "" /31 | 255.255.255.254 | 2 "" /32 | 255.255.255.255 | 1

Note that the /32 subnet mask actually points to only one host, and /31 subnet is simply useless as there are no addresses left for hosts.

Practice Exercises

  1. Determine subnet identifier, broadcast address and number of valid host addresses having: 192.168.86.2/24, 10.0.9.7/26, 192.168.159.8/25, and 172.16.0.189/18.

More information

http://www.ipprimer.com/bitbybit.cfm