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Anatomy of an IP packet

Anatomy of an IP packet
10.1.5 IP packets consist of the data from upper layers plus an IP header. This page will discuss the information contained in the IP header:


• Version – Specifies the format of the IP packet header. The 4-bit version field contains the number 4 if it is an IPv4 packet and 6 if it is an IPv6 packet. However, this field is not used to distinguish between IPv4 and IPv6 packets. The protocol type field present in the Layer 2 envelope is used for that.

• IP header length (HLEN) – Indicates the datagram header length in 32-bit words. This is the total length of all header information and includes the two variable-length header fields.
• Type of service (ToS) – 8 bits that specify the level of importance that has been assigned by a particular upper-layer protocol.
• Total length – 16 bits that specify the length of the entire packet in bytes. This includes the data and header. To get the length of the data payload subtract the HLEN from the total length.
• Identification – 16 bits that identify the current datagram. This is the sequence number.
• Flags – A 3-bit field in which the two low-order bits control fragmentation. One bit specifies if the packet can be fragmented and the other indicates if the packet is the last fragment in a series of fragmented packets.
• Fragment offset – 13 bits that are used to help piece together datagram fragments. This field allows the previous field to end on a 16-bit boundary.
• Time to Live (TTL) – A field that specifies the number of hops a packet may travel. This number is decreased by one as the packet travels through a router. When the counter reaches zero the packet is discarded. This prevents packets from looping endlessly.
• Protocol – 8 bits that indicate which upper-layer protocol such as TCP or UDP receives incoming packets after the IP processes have been completed.
• Header checksum – 16 bits that help ensure IP header integrity.
• Source address – 32 bits that specify the IP address of the node from which the packet was sent.
• Destination address – 32 bits that specify the IP address of the node to which the data is sent.
• Options – Allows IP to support various options such as security. The length of this field varies.
• Padding – Extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits.
• Data – Contains upper-layer information and has a variable length of up to 64 bits.

While the IP source and destination addresses are important, the other header fields have made IP very flexible. The header fields list the source and destination address information of the packet and often indicate the length of the message data. The information for routing the message is also contained in IP headers, which can get long and complex

This page concludes this lesson. The next lesson will focus on IP routing protocols. The first page provides a routing overview.

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