ARP resolves the mapping issue between two different size protocols.
It allows to MAP an IP(32 bit) and MAC(48 bit) together.
How does it work in a nutshell :
Let’s assume that we have two hosts on different networks – A and B, we want them to communicate with each other, what will happen when host A will try reaching the host B for the first time (arp cache empty) :
- As we can determine by ip and subnet mask combination that host B is on another network , Host A will send a broadcast packet which will contain the following fields :
- HW : 1 (Ethernet)
- Protocol : 0x0800 IP
- HW length : (6) 48 bit
- Protocol length : (4) 32-bit
- OPCode : 1 (Request)
- H/W Source : aa:aa:aa:aa:aa:aa – MAC of host A
- Protocol source : 192.168.0.2 – IP of host A
- HW Dest – ff:ff:ff:ff:ff:ff – MAC of broadcast address
- Protocol dest 192.168.0.1 – ip of gateway
Gateway will respond with ARP reply
- HW : 1 (Ethernet)
- Proocol : 0x0800 IP
- HW length : 6 48 bit
- Protocol length : (4) 32-bit
- OPCode : 2 (Reply)
- H/W Source : gg:gg:gg:gg:gg:gg – Gateways MAC
- Protocol source : 192.168.0.1 – IP of gateway
- HW Dest – : aa:aa:aa:aa:aa:aa
- Protocol dest : 192.168.0.2 – IP of host A
The same will happen from other side – if gateway has no ARP entry for host B it will broadcast the ARP request to same broadcast domain(let’s assume we are using only one router) it will get a response from the host and will save its address to arp cache.
After request reply exchange we will have the gateway in our arp-cache now we can send the packets to B using the gateway.
- Host A will send a packet to default gateway with destination MAC of default gateway and destination IP of host B.
- Before sending the packet further gateway will change the source MAC to its own, destination IP will be left the same.
- Host B will reply, MAC of gateway will be the dest MAC, and ip of host A will be dest IP.
We need not to forget about :
Gratuitous ARP – in case if machine is changing it’s MAC or services are being moved, we can have a wrong data cached in ARP caches of devices, to update it machine can send gratuitous arp requests.
NPLC 