ARP question

[robbo1962]

Hi all, i understand the concept of what the ARP does in that it resolves IP addresses to MAC addresses, each network device does this as the MAC address is required to send data. (feel free to correct me if i'm wrong). But what happens if your computer is behind a proxy server or your network is using the NAT facility? how can a particular computer be directly accessed from another one on a different network? Thanks Gary

[hbroomhall]

ARP is only used on a local Ethernet network.

The moment you go through a router or equivalent then things are done another way.

If you are behind a NAT router, for example, your 'default gateway' will be that router. Your box uses ARP to get the MAC address for the router and the packets are sent there. The router then inspects the IP layer (layer 2 is removed) and decides that the packet is to go to the Internet. It then sends it down the interface for the Internet (in a home situation this will be the default route for the router). In most ADSL home router setups this will be a point-to-point link, so ARP wouldn't be used.

Harry.

[robbo1962]

just re read your reply Harry, the pennys dropped. Thanks again Gary

[Stoney]

Using the Internet Protocol, or IP as it is more commonly known!

[hbroomhall]

Using a router - which inspects the IP destination addresses, and then looks in its routing table to see where to send the packet.

Conceptually - inside a router there is no layer 2, as different ports on the router might use differing layer 2 protocols, e.g. Ethernet or PPP.

Harry.

[hbroomhall]

Originally Posted by robbo1962

just re read your reply Harry, the pennys dropped. Thanks again Gary

Ah good! (Our messages crossed I see...)

Harry.