A IEEE 802 MAC address generator

Purpose

This generator can be used to create IEEE 802 conform local administered MAC addresses

get it

go get github.com/c-seeger/mac-gen-go

Usage

package main

import (
  "fmt"
  "net"

  gm "github.com/c-seeger/mac-gen-go"
)

func main() {

  // generate a random local administered unicast mac prefix
  prefix := gm.GenerateRandomLocalMacPrefix(true)

  // calculates the NIC Sufix by ip address
  sufix, err := gm.CalculateNICSufix(net.ParseIP("129.168.12.127"))
  if err != nil {
    // your error handling here
  }
  mac := fmt.Sprintf("%s:%s", prefix, sufix)

  fmt.Println(mac)
}

see examples for more information

testing and coverage

mage test
mage coverage

Background

general stuff

a /8 net has 16,777,216 addresses
a mac address has 6 octets (2^48 addresses)
the first 3 octets are the OUI (Organisationally Unique Identifier)
and the last 3 octets are the NIC (Network interface controler specific)
every octet has 8 bit
[b7][b6][b5][b4][b3][b2][b1][b0]
the first octet has special meaning for b0 and b1
b0 -> 0: unicast, 1: multicast
b1 -> 0: globally unique (OUI), 1: locally administered
to generate a mac address without having a OUI
b0 ->0
using only the last 3 octets (16^6 = 2^24 addresses = 16777216 = /8 net)
is enough to create mac addresses for all ip addresses
per net the first 3 octets need to be generated as a new prefix

prefix block generation

generating a prefix block of 3 octets

first octet in binary xxxxxx10 ( or xxxxxx11 if you want a multicast address)
second and 3rd octet can be generated freely
8 bit block in binary
[0|1] [0|1] [0|1] [0|1] [0|1] [0|1] [0|1] [0|1]
left 4 bits does not matter since they do not effect the b0 and b1 bit
to have b0 -> 0 and b1 -> 1 (a LA unicast address)
[0|1] [0|1] 1 0
so we have sth + 2
where sth is ether 0 (0 0) or 8 (1 0) or 4 (0 1) or 12 (1 1)
so all valid combinations are 2, 6, 10 and 14 which are in hex
2, 6, A, E
the first octet has to be sth like
[0..F][2|6|A|E]
if the address should be multicast
b0 -> 1 and b1 -> 1
so sth + 3
which results in 3, 7, B, F
and the first octet has to be sth like
[0..F][3|7|B|F]
there are 6 free bits in this octet so we have
2^6 permutations for this octet
if we include the other 2 octests (sum up to additonal 16 bits)
we have 2^22 permutations for free local administratered mac addresses (either uni or multicast)
we should exclude some of them because of some registries failed in correctly checking if an OUI has local administraited address (LAA) bit set or not -.-
here is a list https://gist.github.com/aallan/b4bb86db86079509e6159810ae9bd3e4
and checked against http:-standards-oui.ieee.org/oui/oui.txt
to exclude
02 since here are a few registraited ones
AA due to DEC LAA
so we now have
0[3|6|7|A|B|E|F] - 7
[1..9][2|3|6|7|A|B|E|F] - 9*8
A[2|3|6|7|B|E|F] - 7
and
[B..F][2|3|6|7|A|B|E|F] - 5*8
we will lose some of our actual permutations
without the first octet we have 2^16 permutations (remember 2 octest = 8 bit per octet -> 16 bits)
for our first octet we now have 126 possibile permutations left
so we get 2^16*126 = 8257536 possible permutations (this includes multicast)
this is about 49,21% of the full addresspace (2^24) so we lose about half of the addresses
in a perfect world where no registrie failed we would exactly lose 50% (2^16*128 / 2^24) = (2^16 * 2^7 = 2^23) / 2^24 = 0.5
so we lose about 0.79% addresses due to registration failure exclusions
but yeah we do have over 8 million left - show me someone who has more than 8 millions local networks
the registrated private networks are 10.0.0.0/8 (16.77.216) 172.16.0.0/12 (1.048.576) and 192.168.0.0/16 (65.536)
so in the worst case you can cut all these networks into /32 networks with only one ip address so you can have
up to 17891328 networks so our 2^16*126 permutations fit about 46.15% of this worst case szenario or 23.44% when only using unicast

Contribution

Thank you for participating to this project. Please see our Contribution Guidlines for more information.