
106
Appendix B. MAC Addresses usage
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.
FB6000s 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, FB6000 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 FB6000. The subnet status page shows the MAC addresses currently in use on the
Ethernet interfaces.
B.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.
B.1. Using the same MAC address
There are cases where it is sensible or requires 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.
Содержание FB6402
Страница 1: ...FireBrick FB6402 User Manual FB6000 Versatile Network Appliance...
Страница 2: ......