Information Technology Reference
In-Depth Information
IHL (Internet Header Length):
This field is 4 bits in length. It indicates the length
of the header in 32-bit words (4 bytes) so that the beginning of the data can be found
in the IP header. The minimum value for a valid header (five 32-bit words) is 5 (0101).
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To S ( T y p e o f S e r v i c e ) :
This field is 8 bits in length. Quality of service (QoS) param-
eters such as IP precedence or DSCP are found in this field. These are explained fur-
ther in this chapter.
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To t a l L e n g t h :
This field is 16 bits in length. It represents the length of the datagram
or packet in bytes, including the header and data. The maximum length of an IP
packet can be 2
16
- 1 = 65,535 bytes. Routers use this field to determine whether frag-
mentation is necessary by comparing the total length with the outgoing MTU.
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Identification:
This field is 16 bits in length. It identifies fragments for reassembly.
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Flags:
This field is 3 bits in length. It indicates whether the packet can be fragmented
and whether more fragments follow. Bit 0 is reserved and set to 0. Bit 1 indicates May
Fragment (0) or Do Not Fragment (1). Bit 2 indicates Last Fragment (0) or More Frag-
ments to Follow (1).
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Fragment Offset:
This field is 13 bits in length. It indicates (in bytes) where in the
packet this fragment belongs. The first fragment has an offset of 0.
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Time to Live:
This field is 8 bits in length. It indicates the maximum time the packet
is to remain on the network. Each router decrements this field by 1 for loop avoid-
ance. If this field is 0, the packet must be discarded. This scheme permits routers to
discard undeliverable packets.
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Protocol:
This field is 8 bits in length. It indicates the upper-layer protocol. The Inter-
net Assigned Numbers Authority (IANA) is responsible for assigning IP protocol val-
ues. Table 8-2 shows some key protocol numbers. You can find a full list at
www.iana.
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Ta b l e 8 - 2
IP Protocol Numbers
Protocol Number
Protocol
1
Internet Control Message Protocol (ICMP)
2
Internet Group Management Protocol (IGMP)
6
Tran s mis s ion Cont rol P rotocol (TCP)
17
User Datagram Protocol (UDP)
41
IPv6 encapsulation
50
Encapsulating Security Payload (ESP)
51
Authentication Header (AH)
58
ICMPv6
88
