[sllug-members]: mac address

[brian Beck]

ok, thanks, I learned somthing new.

Brian

On 7/15/06, Shaun Kruger <shaun.kruger at gmail.com> wrote:
>
> On 7/14/06, brian Beck <brian at planetbeck.com > wrote:
> > on a discussion on another board, the subject of a isp knowing the mac
> > address of computers behind a firewall came up, It was my understanding
> that
> > the ISP can find out the mac address of computers behind a
> firewall/router,
> > is it possible or am I misunderstanding the tcp/ip protocols?
>
> You can't find out the mac of a computer that is on the other side of
> a router.  When you have a gateway doing NAT the internal addresses
> (192.168.x.x) are not visible from the outside world.  The mac address
> is just the physical hardware address that becomes associated with a
> particular ip address.  You can see this mapping by doing arp -a in
> windows or cat /proc/net/arp in linux.  Ethernet frames know nothing
> of IP addressing.  When you send data over ethernet II it has a 6 byte
> destination mac, a 6 byte source mac (I forget order), and a 2 byte
> protocol type identifier (original ethernet used this 2 bytes as a
> packet length identifier).  The destination mac lets a machine know it
> is supposed to receive or ignore the frame.  It then checks to see if
> it has a handler for the protocol type ID.
>
> If you wanted to find out what machine on an internal network accessed
> a certian site a gateway would have to remember all the external
> addresses that were connected to and what internal addresses requested
> each connection.  It would also have to log the mac address of the
> internal address for each connection (DHCP can assign the same address
> to a new machine with a different mac).
>
> It's not impossible, it's just that no one is doing it.
>
> Shaun
>
> > I.E. a person uses a wireless card to access your wireless lan to lookup
> > porn can the isp tell which computer did the lookup/protection from
> > prosecution
>
>
>
> --
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