Appendix C. MAC Addresses usage

Table of Contents

Ethernet networks use 48 bit MAC addresses. These are globally unique and allocated by the equipment manufacturer from a pool of addresses that is defined by the first three octets (bytes), which identify the organization, and are known as the Organizationally Unique Identifier (OUI). OUIs are issued by the IEEE - more information, and a searchable database of existing OUIs are available at http://standards.ieee.org/develop/regauth/oui/

MAC addresses are commonly written as six groups of two hexadecimal digits, separated by colons or hyphens.

FB2500s currently ship with an OUI value of 00:03:97.

In principle the FireBrick could have a single MAC address for all operations. However, practical experience has led to the use of multiple MAC addresses on the FireBrick. A unique block of addresses is assigned to each FireBrick, with the size of the block dependent on the model.

Most of the time, FB2500 users do not need to know what MAC addresses the product uses. However, there are occasions where this information is useful, such as when trying to identify what IP address a DHCP server has allocated to a specific FB2500. The subnet status page shows the MAC addresses currently in use on the Ethernet interfaces.

C.1. Multiple MAC addresses?

A MAC address does have to be unique on an Ethernet LAN segment, but typically a device will have one MAC address, or one for each physical interface, preset by the network card in use. However, the FireBrick makes use of multiple MAC addresses. There are two key reasons for this.

The FireBrick can operate as a DHCP client device multiple times on the same LAN segment, obtaining several separate IP addresses. This is useful on some cable modem type installations where multiple IPs are only available if the FireBrick appears to be multiple devices at once. Whilst DHCP theoretically does not need separate MAC addresses, experience suggests this is by far the most practical approach. If you have more than one DHCP client subnets in your configuration they will automatically get separate MAC addresses.
In theory the scope of a MAC address is a single LAN segment. The fact that they are globally unique is simply to avoid any clashes on a LAN segment. However, once again, practical experience shows that some network devices and some network switches do not handle the concept of the same MAC address appearing on different ports or VLANs within the network. This can lead to broken networks or traffic leaks between VLANs, neither of which is good. For this reason the FireBrick uses distinct MAC addresses on each interface.

C.1. Using the same MAC address

There are cases where it is sensible or required to use the same MAC address for more than one thing. For a start, the FireBrick does not have unlimited MAC addresses, but there are other reasons, for example:-

Distinct subnets on the same LAN segment do not cause any switch/MAC issues as the FireBrick appears to simply be one device on the LAN segment with multiple IPs. This is quite a normal configuration for network devices. In these cases the FireBrick can use the same MAC address for multiple IPs on the same LAN segment.
There can be MAC restrictions on some devices - this is mainly at the ISP level where peering points and network connections may be set up with limited MAC addresses. In such cases any packet with a different MAC address seen on a port can cause the port to shut down, or the additional MAC addresses to be blocked. For this reason there are cases where multiple subnets need to be restricted to exactly one MAC address.
Tip
The interface settings in the configuration have a restrict-mac setting which, when set to true causes the same MAC to be used for all subnets and operations on that specific interface (port group / VLAN combination).

C.2. Changing MAC address

There is no reason for any network device to maintain the same MAC address for ever. It is normal for the MAC address to change if the network card is changed on a PC, for example.

However, it is inconvenient if MAC addresses change simply because a device is power cycled or a new configuration is loaded. This can cause delays accessing the device if other devices have the MAC cached. It is also a serious problem for ISP links as above where ports are locked to only accept one MAC.

The way the FireBrick manages MAC addresses is designed to be a bit sticky so that a config change will not usually cause a MAC address assigned to a subnet or interface to change.


Appendix B. CIDR and CIDR Notation		C.2. How the FireBrick allocates MAC addresses