IP and ICMP protocols

Posted on October 5, 2009. Filed under: CCNA, ICND1 break down |

Post no longer maintained, updated post is here

IP protocal resides in Network Layer, which provide service to the Transport Layer protocals TCP and UDP.

IP packet structure

bit offset 0–3 4–7 8–15 16–18 19–31
0 Version Header length Differentiated Services Total Length
32 Identification Flags Fragment Offset
64 Time to Live Protocol Header Checksum
96 Source Address
128 Destination Address
160 Options

The header consists of 13 fields, of which 12 are required. The 13th field is optional (red background in table) and aptly named: options.

IP is the primary protocol in the Network Layer (Layer 3) of the OSI Model and has the task of delivering packets from the source host to the destination host solely based on their addresses. IP is considered a best-effort protocol because IP does not provide acknowledgement of the data delivery. Its service can also be characterized as unreliable. In network architectural language it is a connection-less protocol, in contrast to so-called connection-oriented protocols such as TCP and Frame Relay.

ICMP packet structure

The ICMP header starts after bit 160 of the IP header (unless IP options are used). Although ICMP messages are contained within standard IP datagrams, ICMP messages are usually processed as a special case.
ICMP messages are constructed from a normal IP datagram that has generated an ICMP response. IP encapsulates the appropriate ICMP message with a new IP header to get the ICMP message back to the original sending host and transmits the resulting datagram in the usual manner.

Bits 160-167 168-175 176-183 184-191
160 Type Code Checksum
192 ID Sequence

ICND1 and ICND2 break down


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[…] was written by Mike Muuss in December, 1983. It sends a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return. […]

[…] is A Network * The OSI Model * The Data Transmission Process * The TCP/IP Model * TCP And UDP * IP and ICMP protocols * Ports, Sockets and Port Numbers * TCP/IP Applications * Network […]

[…] https://kl2217.wordpress.com/2009/10/05/ip-and-icmp-protocols/ xxxxx http//xyznetwork.blogspot.com/2009/10/ip-and-icmp-protocols_16.html […]

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