TCP/IP Model: Internet Layer

TCP/IP Model: Internet Layer

Much like OSI Model’s 3^rd Layer which is Network Layer, Internet Layer works the same way.

The main reason of Internet Layer Protocol is:

        ·         Routing

        ·         Providing a single network Interface to the upper layer.

IP is a critical part, all network roads lead to IP.

Protocols at the Internet Layer are:

      

       ·         Internet Protocol (IP)

       ·         Internet Control Message Protocol (ICMP)

       ·         Address Resolution Protocol (ARP)

       ·         Reverse Address Resolution Protocol (RARP)

       ·         Proxy ARP

Internet Protocol (IP):

IP is essentially the network layer. Every devices on a network has an IP address.

IP looks at each pakets address. Then, by using routing table, it decides where a packet has to be sent, choosing the best path.

IP receives data segments from the Host-to-Host Layer protocol and fragments them into packets (Datagram). Each packet is assigned the IP address of the sender and the receipent. Each router (Layer 3 device) that receives a datagram makes routing decisions based on the packet’s destination IP Address.

Internet Control Message Protocol (ICMP):

ICMP works at the network layer and is used by IP for many different reasons.

ICMP is a management protocol and messaging service provider for IP.

ICMP packets have following characteristics:

         ·         They provide hosts with information about network problems.

         ·         They are encapsulated within IP Datagram

ICMP is used mainly in following situations:

        1.       Destination Host Unreachable

        2.       Buffer Full: If routers memory buffer is full for receiving incoming datagrams, it uses ICMP to send out this message until congestion is cleared.

        3.       Hops: Each IP packet/datagram is allocated a certain number of hops i.e. the number of routers to pass through, if the Hop reaches its limit the packet expires, ICMP is than used to report about this to the sending host.

        4.       PING (Packet Internet Gropher): Uses ICMP echo request and reply messages to check physical and logical connectivity of machines.

        5.       Traceroute: Using ICMP time-out , traceroute is used to discover the path a packet takes as it traverses an inter-network.

Address Resolution Protocol (ARP):

ARP is used to find out MAC/Hardware address from a known IP address. As a detective agent, ARP interrogates the local network sending out a broadcast asking the machine with the specified IP address to reply with its hardware address.

As we know that MAC and IP Address both are required for a data delivery, ARP helps to find out the MAC address when IP address is known, it sends a broadcast by saying, “hello everyone, I am looking for a MAC address for this IP Address”, this broadcast is received by every device on the broadcast domain, and the device with the IP Address replies to the sender by its MAC address.

So, basically ARP helps to find out the MAC address from a known IP address.

Reverse Address Resolution Protocol (RARP):

RARP resolves MAC address to IP Address, i.e. it is just the opposite of ARP. It is used in deiskless machines, RARP sends out a broadcast with its MAC addressing to RARP server to know its assigned IP address.

Proxy ARP (PARP):

It is useful when default gateway (Router) goes down, PARP helps machines on the subnet to reach remote subnets without configuring routing or even a default gateway.

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