Mac – Are MAC addresses unique when coming out of the factory

mac addressnetwork-adapternetworking

According to Wikipedia:

A Media Access Control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment

But how unique are MAC addresses on devices coming out of the factory? I seem to remember hearing a long time ago about how some manufacturers would reuse MAC addresses on their network cards. Does anyone have any hard facts one way or the other?

Best Answer

There are 2⁴⁸ or 281 474 976 710 656 different potential combinations.

They are reasonably unique.

The first 3 octets define the manufacturer.
The last 3 octets are usually generated at the time of PROM burning. It's up to the manufacturer how they do this.

That obviously gives 16 777 215 possible unique MAC addresses per manufacturer. That's quite alot, so the manufacturer shouldn't re-use one. Some are lazy though, and don't check if they have already allocated a MAC address.

It is quite often possible to change the MAC address using software, so if you do get a duplicate you can map around it.

Related Solutions

Adding virtual interfaces (not VLAN) in linux

You can do this through setting up ethernet bridge. You are effectively setting up a virtual switch/router, and add as many taps/ports as you desire.

I have had the following script setup for running a number of VM in a isolated network. Hope it can be useful to you:

USER=username
NUMBER_OF_VM=4
BR_NAME=vbr
IP_RANGE=10.1.1.1/24
IP_ROUTE=10.1.1.0/24

# Creates a new instance of the ethernet bridge
echo "Creating bridge: $BR_NAME"
brctl addbr $BR_NAME
# Activate the bridge
ip link set up dev $BR_NAME

# Create persistent TAPs and attach to bridge
echo "Creating $NUMBER_OF_VM tap(s) to attach to the $BR_NAME bridge"
NB=0
while [ $NB -lt $NUMBER_OF_VM ]
do
   tunctl -t vbxtap$NB -u $USER
   ip link set up dev vbxtap$NB
   brctl addif $BR_NAME vbxtap$NB
   let NB=$NB+1
done

# Assign ip address and routing to the bridge interface
echo "Assign $IP_RANGE to $BR_NAME"
ip addr add $IP_RANGE dev $BR_NAME
#echo "Assign routing rule $IP_ROUTE to $BR_NAME"
#ip route add $IP_ROUTE dev $BR_NAME

Mac – Identical MAC Addresses on Network

Use a router to create a second subnet for your heatpump (or alternatively, for the Wi-Fi devices). MAC addresses are only relevant within the same L2 network they're on.

If you insist on having everything on a single subnet, the only other option which could help is a switch/bridge with "layer-2 NAT" function, placed like a firewall between the heatpump and the rest of the network. For example a Linux software bridge could probably achieve this via 'ebtables', and most Mikrotik RouterOS devices have L2 NAT under '/interface bridge nat' as well.

Without doing either of the above, trying to keep both devices in the same subnet will be nearly impossible, as every single switch or Wi-Fi access point will be constantly changing its mind about where to deliver the packets, and whenever they get delivered to the wrong device it will kill the unrecognized TCP connection (which is normal behavior).

(Note: Even with two subnets, if you try to do this using VLANs, it can still cause trouble if the network switches don't have IVL...)

Best Answer

Related Solutions

Adding virtual interfaces (not VLAN) in linux

Mac – Identical MAC Addresses on Network

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