Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 215Figure 2.28IP header formatHeader length and total length ®eldsThe header length ®eld follows the version ®eld and is also four bits in length. This®eld indicates the length of the header in 32-bit words. In comparison, the totallength ®eld indicates the total length of the datagram to include its header andhigher layer information. The use of 16 bits for the total length ®eld enables an IPdatagram to be up to 2 16 1 or 65 535 octets in length.Type of service ®eldThe type of service ®eld identi®es how the datagram is handled. Three of the eightbits in this ®eld are used to denote the precedence or level of importance assignedby the originator. Thus, this ®eld provides a priority mechanism for routing IPdatagrams.