Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual download pdf (Page 217)

Networking

217

www.vmware.com

Setting the MAC Address Manually

for a Virtual Machine

VMware ESX Server automatically generates MAC addresses for the virtual network

adapters in each virtual machine. In most cases, these MAC addresses are appropriate.

However, there may be times when you need to set a virtual network adapter’s MAC

address manually — for example:

• You have more than 256 virtual network adapters on a single physical server.

• Virtual network adapters on different physical servers share the same subnet and

are assigned the same MAC address, causing a conflict.

• You want to ensure that a virtual network adapter always has the same MAC

address.

This document explains how VMware ESX Server generates MAC addresses and how

you can set the MAC address for a virtual network adapter manually.

How VMware ESX Server Generates MAC Addresses

Each virtual network adapter in a virtual machine gets its own unique MAC address.

ESX Server attempts to ensure that the network adapters for each virtual machine that

are on the same subnet have unique MAC addresses. The algorithm used by ESX

Server puts a limit on how many virtual machines can be running and suspended at

once on a given machine. It also does not handle all cases when virtual machines on

distinct physical machines share a subnet.

A MAC address is a six-byte number. Each network adapter manufacturer gets a

unique three-byte prefix called an OUI — organizationally unique identifier — that it

can use to generate unique MAC addresses. VMware has two OUIs — one for

automatically generated MAC addresses and one for manually set addresses.

The VMware OUI for automatically generated MAC addresses is 00:05:69. Thus the first

three bytes of the MAC address that is automatically generated for each virtual

network adapter have this value. ESX Server then uses a MAC address generation

algorithm to produce the other three bytes. The algorithm guarantees unique MAC

addresses within a machine and attempts to provide unique MAC addresses between

ESX Server machines.

The algorithm that ESX Server uses is the following:

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Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual Page 217