| Rev | Author | # | Line |
|---|---|---|---|
| 1 | PerryLorier | 1 | [Acronym] for __M__edia __A__ccess __C__ontrol. |
| 2 | |||
| 4 | JohnMcPherson | 3 | The term __MAC Address__ is used to refer to the hardware address of an Ethernet card. These are usually hard-coded in an __Address [ROM]__ on the network interface card. |
| 1 | PerryLorier | 4 | |
| 4 | JohnMcPherson | 5 | This is the LinkLayer; an [Ethernet] frame header contains the both the source and destination [Ethernet] MAC addresses and a [Protocol] (or Length) field. |
| 1 | PerryLorier | 6 | | from hardware address |
| 7 | | to hardware address | ||
| 8 | | protocol field | ||
| 9 | | <data> | ||
| 2 | JimCheetham | 10 | |
| 11 | |||
| 4 | JohnMcPherson | 12 | A MAC address is 6 bytes (48) bits long, and is commonly represented as a colon-delimited sequence of 12 hex digits:"ab:cd:ef:gh:ij:kl". |
| 13 | |||
| 14 | The first 3 bytes (the "ab:cd:ef") identify the manufacturer of the network card, as overseen by the [IEEE] and [IANA]. See [http://standards.ieee.org/regauth/oui/oui.txt] for more information. | ||
| 15 | |||
| 16 | A MAC address should be globally unique - no two network cards should ever have the same MAC address, although some low-quality hardware manufacturers have been known | ||
| 17 | to distribute [NIC]s with identical MAC addresses before. Some cards (notably, those used with Solaris machines, but also some cards supported under Linux) may be software-reprogrammable. | ||
| 18 | |||
| 19 | Since everyone's network card is (in theory) unique, this is often used as | ||
| 20 | a unique identifier for a machine (even though a machine may have multiple | ||
| 21 | network cards, or not have one at all). | ||
| 3 | PerryLorier | 22 | |
| 23 | The low nybble of the first byte (the "b") contains a lot of information, the lowest bit of this means that the traffic is multiple destination (ie, multicast or broadcast). | ||
| 24 | This is because on the wire each byte is sent low bit first. | ||
| 25 | |||
| 26 | |||
| 4 | JohnMcPherson | 27 | |
| 28 | |||
| 29 | You can find out the MAC address of your network cards using the ifconfig(8) utility. For example in Linux: | ||
| 30 | $ /sbin/ifconfig | ||
| 31 | eth0 Link encap:Ethernet HWaddr __00:07:A9:11:40:A8__ | ||
| 32 | ... | ||
| 33 | |||
| 34 | It may look different in other [Unix] variants, [OpenBSD] shows the following: | ||
| 35 | $ /sbin/ifconfig -a | ||
| 36 | rl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500 | ||
| 37 | address: __00:05:1d:9b:f1:10__ | ||
| 38 | |||
| 3 | PerryLorier | 39 | |
| 40 | ---- | ||
| 41 | Recognising various mac addresses: | ||
| 42 | * http://cert.uni-stuttgart.de/archive/intrusions/2004/01/msg00058.html | ||
| 43 | |||
| 44 | Some common mac addresses you'll see (And get confused about): | ||
| 45 | |||
| 46 | |1:0:c:cc:cc:cc|Cisco discovery Protocol (CDP) | ||
| 47 | |1:0:c:0:0:0|Spanning Tree Protocol (STP) | ||
| 48 | |1:0:c:0:0:0|Cisco Interswitch Link | ||
| 49 | |1:0:5e:x:x:x|IP Multicast (RFC:1112) | ||
| 4 | JohnMcPherson | 50 | |
| 51 | ---- | ||
| 52 | CategoryNetworking |
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