TELECOM Digest OnLine - Sorted: Re: Change MAC Address Can Change IP Address of a Machine?

Re: Change MAC Address Can Change IP Address of a Machine?

Gordon Burditt (gordonb.ian3f@burditt.org)
Thu, 29 Sep 2005 18:33:34 -0000

> Is there any way to change the MAC address of a machine?

Sometimes, depending on the network card and the OS software.
For example, in FreeBSD you might use:

ifconfig de0 ether 01:23:45:67:89:ab

I understand Linux has a similar ability, and I think Windows XP can
do it in a much more GUI-ish way. Some routers have a "clone MAC
address" ability. This won't change the MAC address kept in the ROM,
so the command has to be re-done every time the machine boots.

> IP address is assigned by ISP. My understanding is that IP address
> is based on the MAC address of a machine, is that correct concept?

When using DHCP or BOOTP, the IP address may be assigned based on the
MAC address due to the configuration of the DHCP server (or it might
just take an available currently-unused one from the pool). Note that
an unrecognized MAC address might be denied service entirely rather
than given a different IP. This depends entirely on how your ISP set
up the DHCP server.

If a machine is manually assigned an IP address or it is being
assigned an IP address from a DHCP server that YOU control, you may
assign any IP address. There's no magic formula that you can use to
calculate an IP address from a MAC address.

Gordon L. Burditt

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