Arp

ARP finds the hardware address of a host from a known IP address. Here's how it works: When IP has a datagram to send, it must inform a Network access protocol, such as Ethernet, of the destination's hardware address on the local network. (It has already been informed by the upper-layer protocols of the destination's IP address). If IP doesn't find the destination host's hardware address in the ARP cache, it uses ARP to find this information.

From the text above what does it mean when it says 'It has already been informed by the upper-layer protocols of the destination's IP address'

Also how does a host know what IP address to use when sending a datagram to its destination, which is probably a completely different question.

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Comments

bermovick

Oh boy, let's see if I can get this right.

The best way I learn is by doing an example, so we'll do that.

You type in Google into your web browser right after turning on your
computer.

Your computer doesn't know what that means, so the upper layers hand it down to
the transport layer's DNS protocol. Your DNS server's IP address was something you either
entered or was provided through DHCP. So here the network layer knows of the
destination's IP address because of being manually entered or because of DHCP.

DNS will return that Google is 74.125.45.147, which the transport layer
will return upwards to the application so the application can then start handing
the data down the stack with the IP address your data is destined for.

networker050184

The application is responsible for determining where the traffic needs to go. For instance if you click on a link in your browser thats how your computer knows which IP/hostname to send the traffic to. Layer2/3's responsibility is to get the traffic there once the application knows where it wants to go.

Damn you beat me to it. One thing to add though, DNS is an application and does not reside on the transport layer.

Bolton07

Thanks thats really helpful

Did that cover the first question too -

From the text above what does it mean when it says 'It has already been informed by the upper-layer protocols of the destination's IP address'

networker050184

Bolton07 wrote: »

Thanks thats really helpful

Did that cover the first question too -

From the text above what does it mean when it says 'It has already been informed by the upper-layer protocols of the destination's IP address'

Yes, the web browser being the application layer that is sending this information to the lower-layers. The browser says go to 1.1.1.1 TCP 80. Thats when the network stack takes over.