[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: Network Problems

On Thu, Mar 22, 2018 at 10:50:59AM +0000, David wrote:
On Thu, 2018-03-22 at 11:15 +0100, tomas@tuxteam.de wrote:
On Thu, Mar 22, 2018 at 09:46:02AM +0000, Thomas Pircher wrote:
> On Thu, 22 Mar 2018, David wrote:
> > Does Debian keep a table of MAC addresses? If so where can I locate it?
> The kernel does. You can get the ARP table e.g. with
>     ip neigh list

There's also the arp (8) command: you can query and manipulate
the ARP table (arp stands for "address resolution protocol", which
is tasked with keeping this MAC -- IP mapping up to date).

For example, just typing "sudo arp" will show you a list of known
IP Address to MAC address mappings. With "arp -s <ip> <mac>" you
can set one such mappings. And so on.

-- tomás
Thank you to Thomas and Tomas for replying.

I think as Thomas stated it's a router problem, I've been looking at
what the various routers are storing and not found a problem to date.

I suspect that allocating random MAC addresses is the problem and I need
to find out what I can allocate without causing problems. There must be
some rules somewhere, I suspect the Arduino uses a block of addresses
and I've duplicated something.

MAC addresses follow a set format. They are 6 bytes long: the first 3 bytes is the Organisationally Unique Identifier (OUI), the second 3 bytes are the NIC specific identifier.

Typically, MAC addresses are set by the manufacturer of the Ethernet device. Each manufacturer is allocated one or more OUIs. They are free to allocate the last three bytes however they like, so long as they remain unique within the organisation. So Intel can produce a device with 12:34:56 as the last three bytes, and so can Netgear, but once combined with the OUI, the whole *should* be globally unique.

So, what about locally-created Ethernet devices (e.g. Virtual Machine interfaces, or devices without a burned-in MAC address)? For these, you don't need to apply for your own OUI. The MAC address standard states that if the second-least-significant bit of the first octet is 1, then the whole MAC address is "locally administered". Thus, if your MAC address starts with "x2", "x6", "xA" or "xE" (where x is any digit), then it is locally-adminsitered and, in theory, it is up to you to ensure uniqueness (at least, unique within the boundary of your Ethernet domain).


For more information, please reread.

Attachment: signature.asc
Description: PGP signature

Reply to: