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Newer page: version 4 Last edited on Tuesday, October 12, 2004 2:04:11 pm by JohnMcPherson
Older page: version 3 Last edited on Thursday, September 23, 2004 5:43:58 pm by PerryLorier Revert
@@ -1,23 +1,42 @@
 [Acronym] for __M__edia __A__ccess __C__ontrol. 
  
-This is the LinkLayer, and is usually talked about in [Ethernet ] where it has a very simple structure:  
+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.  
  
+This is the LinkLayer; an [Ethernet] frame header contains the both the source and destination [Ethernet] MAC addresses and a [Protocol] (or Length) field.  
 | from hardware address 
 | to hardware address 
 | protocol field 
 | <data> 
  
-The term __MAC Address__ is also 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.  
  
-A MAC address is 48 bits long, and is commonly represented as a colon-delimited sequence of 12 hex digits:"ab:cd:ef:gh:ij:kl". 
+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".  
+  
+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.  
+  
+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  
+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.  
+  
+Since everyone's network card is (in theory) unique, this is often used as  
+a unique identifier for a machine (even though a machine may have multiple  
+network cards, or not have one at all)
  
 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). 
 This is because on the wire each byte is sent low bit first. 
  
-The first 24 bits/6 digits (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.  
  
-A MAC address should be globally unique - no two network cards should ever have the same MAC address. Note that some network cards (notably, those used with Solaris machines ) may be software -reprogrammable.  
+  
+  
+You can find out the MAC address of your network cards using the ifconfig (8 ) utility. For example in Linux:  
+ $ /sbin/ifconfig  
+ eth0 Link encap:Ethernet HWaddr __00:07:A9:11:40:A8__  
+ ...  
+  
+It may look different in other [Unix] variants, [OpenBSD] shows the following:  
+ $ /sbin/ifconfig -a  
+ rl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500  
+ address: __00:05:1d:9b:f1:10__  
+  
  
 ---- 
 Recognising various mac addresses: 
 * http://cert.uni-stuttgart.de/archive/intrusions/2004/01/msg00058.html 
@@ -27,4 +46,7 @@
 |1:0:c:cc:cc:cc|Cisco discovery Protocol (CDP) 
 |1:0:c:0:0:0|Spanning Tree Protocol (STP) 
 |1:0:c:0:0:0|Cisco Interswitch Link 
 |1:0:5e:x:x:x|IP Multicast (RFC:1112) 
+  
+----  
+CategoryNetworking