Packet flow in the same Network

• Difficulty Level : Basic
• Last Updated : 25 Oct, 2021

Prerequisite – How ARP works? 
To transfer a packet from source to destination, both the MAC address and IP address of the destination should be known. If the destination MAC address is not present then ARP will resolve this issue first then the packet will be delivered to a destination host. 

There are simple rules for a packet flow in a network: 

1. If the destination host is present in the same network as the source host then the packet will be delivered directly to the destination host using MAC address.
2. Within a network, the packet will be delivered on the basis of MAC address.
3. MAC address never crosses its broadcast domain.

Now, first, we have to take an idea about ARP. 

Address Resolution Protocol – 
Address Resolution Protocol is a layer 2(Data link layer) protocol that is used to find the MAC address of the known IP address. 

There are some important terms associated with ARP: 

ARP cache is a table maintained by ARP that contains an IP address with its associated MAC address and type. If MAC address is learned dynamically then the type will be dynamic and if MAC address is added manually then the type will be static. 

ARP request is a broadcast message generated by the source to find the destination MAC address if the ARP is not resolved initially. 

ARP reply is a unicast message from destination to source device containing the destination MAC address. 

Explanation –  

The steps included in the ARP process are as follows:- 

When a source wants to send a packet to the destination device then, 

1. The source ARP cache is checked if the ARP is resolved or not. If the ARP is not resolved, it puts the packet on hold and generates an ARP request. 

2. If the ARP is already resolved then the packet will be delivered to the destination host.

3. The ARP request is broadcast all over the network to find out the device has a destination IP address. 
Note – If the destination is present in the same network then ARP will find out destination MAC address but if it is present in a different network then ARP will find out default gateway MAC address. 

4. When the device having the destination IP address receives the ARP request, it updates its own ARP cache.

5. The destination host machine generates an ARP reply containing its own MAC address.

6. Now, the device having the source IP address receives the ARP reply and updates its ARP cache. 

7. Since, both source and destination IP address and MAC address are available now, therefore, the packet is delivered to the destination host.

Now, we have taken an idea about the ARP protocol. Let’s see about the packet flow. 

Now we will understand how the packet is delivered to the destination when the destination is present in the same network(network of the source). 

Here is the topology in which host A has IP address 192.168.1.1, host B has IP address 192.168.1.2, and the router has IP address 192.168.1.3 on interface fa0/0. 

Now how to source device will know that the destination is present in the same or different network. Let us understand:- 

AND operation is performed between the source IP address, source subnet mask and destination IP address, source subnet mask. If the resultant of both are the same then the destination is present in the same network otherwise in a different network. 

Let us try to ping host B from host A. 

As you can see 2 packets are generated, one of ICMP and the other of ARP(green). ARP frame is generated because host A has not yet communicated to host B i.e. the ARP has not been resolved i.e ARP will be resolved first so that host A has an entry for host B MAC address. 

As already explained the ARP request will be broadcast first for the target IP address within the network because routers do not forward broadcast packets. The broadcast request is received by the switch as shown in the above figure. 

The switch broadcasts the ARP request as the entry in the ethernet header is FFFF.FFFF.FFFF (broadcast MAC address). 

The request is received by Host B as shown in the above figure. Host B generates an ARP reply immediately specifying its own MAC address. 

Now the host B unicast the ARP reply to host A which is received by the switch which in turn forward it to host A as shown in the above 2 figures. 

Note – 
The switch is able to unicast the reply because the switch has put an entry for host A in its MAC table when hosting A broadcasts the ARP request.in the same way, a switch has also put an entry for the host B when the switch receives

 the ARP reply. 

Now the ARP has been resolved and the ICMP will be unicast to the host B from host A(as shown above).  

Now the ICMP acknowledgment packet will be unicast from host B to host A i.e. host B is successfully pinged from host A as shown in the above figures.