arp

[carl_townshend]

when do pc's need to arp for another address, is it every time it needs to talk to that other pc ?

[mark.edwards]

generally they will send arp request for devices on the same network only. For devices on different networks they will arp for the local gateway address.

[carl_townshend]

is this every time they need to speak to another machine ?

[mark.edwards]

they will need to arp each time they need to talk to another machine. One they have an entry in the arp cach for a host they will not need to arp again unless the entry is timed out.

[carl_townshend]

hi there, If then my pc finds an ip address and asks for its mac address to switch the data, if its already found what ip address it has why does it need to reply with its mac address ?

[a.hajhamad]

Hi,

When your PC needs to talk with ip address within its subnet for the first time, the switch which this PC is connected broadcast this request to its all ports in order to get the destination ip address, the destination PC respons with its mac address and the switch store the mac-address with the port which this PC is connected in its table which is called CAM table.

Then your PC get that mac and store it in its ARP table.

And also your PC sends its mac address in order for the switch add it in its CAM table.

If your PC needs to talk with another IP address (not in its subnet), asks (query) for its default gateway mac address to forward the packet.

please rate if it helpful!

Thanks

Abd Alqader

[carl_townshend]

so your saying the switch will actually look for the ip address first, does the switch store this infomation as a mac to ip address mapping ?

[a.hajhamad]

No, the switch store the mac address with port number which this mac originated. since the switch is layer2.

For example:

Your PC has an IP address:1.1.1.150/24 which is connected to port 0/1

and you need send a packet to 1.1.1.160/24, the switch will broadcast this request to all ports except the port 0/1 to see who is the owner of IP 1.1.1.160/24, examine that this PC is located at port 0/5, the PC will send a reply to the broadcast request, the switch store the IP's 1.1.1.160/24 mac address with port 0/5

imagine that PC with IP 1.1.1.160/24 has a mac: 11.22.33.44.55.66

And your PC 1.1.1.150/24 has a mac 66.55.44.33.22.11

the cam table will look like this:

0/5 11.22.33.44.55.66

0/1 66.55.44.33.22.11

i hope this is fine

Thanks

Abd Alqader