ZyXEL Communications ZyWALL 300 User Manual

 Chapter 29 IDP

ZyWALL USG 300 User’s Guide

Figure 329   IP v4 Packet Headers

 The header fields are discussed below:  

Table 135   IP v4 Packet Headers 

Version

The value 4 indicates IP version 4.

IHL

IP Header Length is the number of 32 bit words forming the total 
length of the header (usually five).

Type of Service

The Type of Service, (also known as Differentiated Services Code 
Point (DSCP)) is usually set to 0, but may indicate particular quality 
of service needs from the network.

Total Length

This is the size of the datagram in bytes. It is the combined length 
of the header and the data. 

Identification

This is a 16-bit number, which together with the source address, 
uniquely identifies this packet. It is used during reassembly of 
fragmented datagrams.

Flags

Flags are used to control whether routers are allowed to fragment 
a packet and to indicate the parts of a packet to the receiver.

Fragment Offset

This is a byte count from the start of the original sent packet.

Time To Live

This is a counter that decrements every time it passes through a 
router. When it reaches zero, the datagram is discarded. It is used 
to prevent accidental routing loops.

Protocol

The protocol indicates the type of transport packet being carried, 
for example, 1 = ICMP; 2= IGMP; 6 = TCP; 17= UDP. 

Header Checksum

This is used to detect processing errors introduced into the packet 
inside a router or bridge where the packet is not protected by a link 
layer cyclic redundancy check. Packets with an invalid checksum 
are discarded by all nodes in an IP network.

Source IP Address

This is the IP address of the original sender of the packet.

Destination IP Address

This is the IP address of the final destination of the packet.