Specifications

Administration Guide
312 VMware, Inc.
How VMware ESX Server Generates MAC Addresses
Each!virtual!network!adapter!in!a!virtual!machine!gets!a!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!
limits!the!number!of!virtual!machines!that!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!tw o!OUIs:!one!for!automatically!
generated!MAC!addresses!and!one!for!manually!set!addresses.!One!OUI!(00:0C:29)!is!
used!only!for!generated!addresses!and!the!other!OUI!(00:50:56)!is!used!for!both!
generated!and!manually!set!addresses.
Because!the!VMware!
OUI!for!generated!MAC!addresses!is!00:0C:29,!the!first!three!bytes!
of!the!MAC!address!that!is!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!to!generate!MAC!address!is!the!following:!
ESX!Server!uses!the!VMware!UUID!(Universally!Unique!Identifier)!to!generate!MAC!
addresses!and!then!checks!for!any!conflicts.!If!there!is!a!conflict,!an!offset!is!
added!and!
it!is!checked!again,!until!there!is!no!conflict.!(The!VMware!UUID!is!based!on!the!path!
to!the!virtual!machine!and!the!hostʹs!SMBIOS!UUID.)
After!the!MAC!address!has!been!generated,!it!does!not!change,!unless!the!virtual!
machine!is!moved!to!a!different!location,!for!
example,!a!different!path!on!the!same!
server!or!a!different!ESX!Server!machine.!ESX!Server!saves!the!MAC!address!in!the!
configuration!file!of!the!virtual!machine.!
ESX!Server!keeps!track!of!all!MAC!addresses!that!have!been!assigned!to!network!
adapters!of!running!and!suspended!virtual!machines!on!a
!given!physical!machine.!ESX!
Server!ensures!that!the!virtual!network!adapters!of!all!of!these!virtual!machines!have!
unique!MAC!addresses.!
The!MAC!address!of!a!powered‐off!virtual!machine!is!not!checked!against!running!or!
suspended!virtual!machines.!ESX!Server!does!not!keep!track!of!your!predetermined!
generated!MAC!
addresses.!So!you!can!have!multiple!NICs!(of!the!same!virtual!machine!
N
OTE Addresses!generated!by!Virtual!Center!or!by!VMware!GSX!Server!are!in!the!00:50:56!
range.