INTERNET LAYER
The Internet Protocol (IP) is the most important protocol of the Internet layer. All traffic, incoming and out going, goes through IP. The primary purpose of the Internet layer is to route packets between different hosts which is accomplished by the addressing scheme of IP. The Internet consists of four protocols, as shown in TABLE 3:

Protocol	Description
Internet Protocol (IP)	The internet protocol (IP) provides datagram service between hosts. It is responsible for addressing packets, packet routing, fragmentation, and reassembly, and moving data between the Transport and Network Access layers.
Internet Control Message Protocol (ICMP)	The Internet Control Message Protocol (ICMP) lets routers and hosts send error or control messages to other routers or hosts.
Address Resolution Protocol (ARP)	The Address Resolution Protocol (ARP) translates a host's software address to a hardware address.
Reverse Address Resolution Protocol (RARP)	The Reverse Address Resolution Protocol(RARP) determines a software address from a hardware address. Diskless workstations often use this protocol when booting up to find out what their IP address will be. DHCP, which is a superset of BOOTP, is more commonly deployed than RARP for this functionality these days.

TABLE 3: DoD MODEL IONTERNET LAYER PROTOCOLS.

INTERNET PROTOCOL

IP is a connectionless protocol and does not guarantee delivery of packets across a network. IP relies on higher-layer protocols either in the Transport or Application layers to provide connection- oriented service if necessary. (In some older implementations, the Data Link layer is used. The IP header contains many fields, the most important of which are the source address, Destination address, and Time to Live.
Delivering a packet from one machine on the internetwork to another is handled by IP. For further information on IP, see this link.