Learning by Doing: CISCO Certified Network ... - SCN Research

The Type-of-Service field is 8 bits long. The first three bits are not used anymore. The
next four are the “type of service” bits and the last bit is always set to zero because it is
not used. Only one of the four “type of service” bits can be set to a one at a time while
all other bits are set to zero. These indicate what type of service will be performed. The
types of service are given by:
Type of Service Binary Hexadecimal
Normal service 0000 0x00
NNTP (Usenet news) 0001 0x02
IGP/SNMP 0010 0x04
FTP data//SMTP data/ 0100 0x08
DNS zone xfr/
Telnet/Rlogin/DNS UDP
query/SMTP command
phase/TFTP
1000 0x10
The Length field is the length of the IP datagram portion in bytes (maximum size of
65536 bytes).
The Identification field contains a unique number for each sent packet. It is 16 bits and
given in hexadecimal.
The Flags field uses one bit of it’s 3 bits to identify that “this packet is part of a larger
packet that has been fragmented.”
The Fragment Offset field contains the extra information required with a fragmented
packet. The last of this 13-bit field is able to tell the sending node to “never fragment the
packet.” If fragmentation is needed and this bit is set it will generate an error message
and the information will not be processed. Ahh…playground of the hackers.
The Time to Live (TTL) field sets the maximum number of hops (or routers) that the
packet can pass through on the way to its destination.
The Protocol (Prot) field shows which protocol was used to encapsulate and create the
data. This field is 8 bits long.
The Header Checksum (HC) is an error control mechanism for this point to the end of
the data field. It is 16 bits long.
The Source Address (SA) is the logical address (IP) of the networking device sending the
information. This is 32 bits in hexadecimal. Notice how in IP the source address comes
before the destination address.
The Destination Address (DA) is the logical address (IP) of the networking device the
information is going to be sent to. This is 32 bits in hexadecimal.
79