#240 IPv4 Addresses with Ruby

All about dissecting IPv4 addresses with ruby including networks, netmasks and CIDR

Notes

IP addresses are understood for routing purposes according to the Classless Inter-Domain Routing (CIDR) strategy as specified in RFC1519.

While this also applies to IPv6 addresses, I’ll just consider IPv4 addresses here.

CIDR with IPv4

A given IPv4 addresses (e.g. 12.13.14.15) has two parts:

network prefix: defined by the most significant bits. This identifies a whole network or subnet.
host identifier: defined by the least significant bits. This specifies a particular interface of a host on that network.

The subnet mask defines the boundary between network prefix and host identifier.

For example:

198.51.100.14/24 is CIDR notation meaning
- IP address = 198.51.100.14
- and 24 bits of network prefix
24 bits of network prefix is ofter written as the equivalent subnet netmask 255.255.255.0
that leaves 8 bits for host identifiers within the network
- i.e. 256 host identifiers / IP addresses
the first IP in the network is sometimes referred to as the “network address”
- i.e. 198.51.100.0 in this case
the last IP in the network is used as a broadcast address for the network
- i.e. 198.51.100.255 in this case
that leaves 254 addresses to actually be used for host interfaces in the network

The Subnet Calculator site is an easy way to test/check IP addresses

Dissecting an IPv4 Address with Ruby

The IPAddr standard library provides the basic tools for dissecting and IPv4 address (with a bit of finagling).

The ipv4info.rb script provides an example of how to parse and use an IPv4 address. Some example runs..

Given a network address and CIDR mask, it correctly extracts the network and host information:

$ ./ipv4info.rb 198.51.100.14/24
Given: 198.51.100.14/24, inspected using the IPAddr stdlib..

               IPv4? : true
            Private? : false
                CIDR : 24
         Subnet mask : 255.255.255.0
     Network address : 198.51.100.0
Addresses in network : 256
    Hosts in network : 254

It recognises private (unquotable) addresses:

$ ./ipv4info.rb 192.168.10.10/28
Given: 192.168.10.10/28, inspected using the IPAddr stdlib..

               IPv4? : true
            Private? : true
                CIDR : 28
         Subnet mask : 255.255.255.240
     Network address : 192.168.10.0
Addresses in network : 16
    Hosts in network : 14

Adjusting the CIDR mask resizes the network accordingly:

$ ./ipv4info.rb 192.168.10.10/30
Given: 192.168.10.10/30, inspected using the IPAddr stdlib..

               IPv4? : true
            Private? : true
                CIDR : 30
         Subnet mask : 255.255.255.252
     Network address : 192.168.10.8
Addresses in network : 4
    Hosts in network : 2

And if no CIDR mask is provided, the address is treated as a “network of 1 host”:

$ ./ipv4info.rb 192.168.10.10
Given: 192.168.10.10, inspected using the IPAddr stdlib..

               IPv4? : true
            Private? : true
                CIDR : 32
         Subnet mask : 255.255.255.255
     Network address : 192.168.10.10
Addresses in network : 1
    Hosts in network : 1

PS: there’s a test suite for the script:

$ ./test_ipv4info.rb
Run options: --seed 51973

# Running:

.....................

Finished in 0.002686s, 7818.3173 runs/s, 7818.3173 assertions/s.

21 runs, 21 assertions, 0 failures, 0 errors, 0 skips

netaddr gem

The IPAddr library has some limitations:

netmask is not directly provided, but can be generated with IPAddr e.g. IPAddr.new('255.255.255.255').mask(cidr).to_s
host and address counts are not directly provided, so need to be calculated

The netaddr gem is a Ruby library for performing calculations on IPv4 and IPv6 subnets. It handles makes most of the IPAddr limitations, with the exception of the private address detection.

See ipv4info_netaddr.rb for a rewrite of the script but using the netaddr gem. It behaves the same, bu needs a bundle first to make sure the gem is installed:

$ bundle install
$ ./ipv4info_netaddr.rb 198.51.100.14/24
Given: 198.51.100.14/24, inspected using the NetAddr gem:

               IPv4? : true
            Private? : false
                CIDR : 24
         Subnet mask : 255.255.255.0
     Network address : 198.51.100.0
Addresses in network : 256
    Hosts in network : 254

PS: there’s also a test suite:

$ ./test_ipv4info_netaddr.rb
Run options: --seed 6405

# Running:

.....................

Finished in 0.002640s, 7954.5452 runs/s, 7954.5452 assertions/s.

21 runs, 21 assertions, 0 failures, 0 errors, 0 skips

Credits and References