IP address longest prefix matching algorithm in ruby

ip_address_longest_prefix_matching.rb

# IP address longest prefix matching algorithm in ruby
# Used manual conversions methods, instead of ruby's built in methods

# Given the router receives a destination ip address "ip"
# It has to find out on which network it should forward it to, from its routing table.
# To find the network we need to do an AND operation of the binary representation of the dest IP and the
#   destination subnet mask of our routing table "ip" AND "dest_mask"

# https://community.cisco.com/t5/switching/please-explain-how-the-longest-prefix-matching-works/td-p/2891235

### routing table example
# | network  | mask     |
# | 10.0.0.0 | 255.0.0.0|

BITS = [128, 64, 32, 16, 8, 4, 2, 1].freeze

def to_bin(int)
  BITS.map do |bit|
    if int < bit
      '0'
    else
      int -= bit
      '1'
    end
  end.join
end

def to_dec_ip(bin)
  ip = []
  bin.scan(/.{1,8}/).each do |bin_group|
    temp = 0
    bin_group.split('').each.with_index { |bit, i| temp += BITS[i] if bit == '1' }
    ip << temp.to_s
  end
  ip.join('.')
end

def ip_to_bin(ip_string)
  ip_string.split('.').map { |ip_group| to_bin(ip_group.to_i) }.join
end

def ip_anding(ip1, ip2)
  ip1.split('').map.with_index { |bit, i| (bit.to_i & ip2[i].to_i).to_s }.join
end

ip = '10.10.10.1' # destination ip address in decimal
dest_mask = '255.0.0.0' # possible network mask in decimal

dest_ip_bin = ip_to_bin(ip) # convert dest ip to binary string
dest_network_ip_bin = ip_to_bin(dest_mask) # convert dest mask to binary string

valid_dest_network_ip_bin = ip_anding(dest_ip_bin, dest_network_ip_bin) # perform an AND operation

# The result is the network address, that should match the routing table's network
# if there are more than 1 networks matching, the longest prefix takes precedence,
# or if the routing is in prefix order, then the first match wins.
puts to_dec_ip(valid_dest_network_ip_bin)