Routers

Routers
Internetworking among dissimilar subnetworks is achieved by using routers to
interconnect the subnetworks. Essential functions that the router must perform
include the following:
1. Provide a link between networks.
2. Provide for the routing and delivery of data between processes on end systems
attached to different networks.
3. Provide these functions in such a way as not to require modifications of the
networking architecture of any of the attached subnetworks.
The third point implies that the router must accommodate a number of differences
among networks, such as the following:
• Addressing schemes: The networks may use different schemes for assigning
addresses to devices. For example, an IEEE 802 LAN uses 48-bit binary
addresses for each attached device; an ATM network typically uses 15-digit
decimal addresses (encoded as 4 bits per digit for a 60-bit address).
Some form of global network addressing must be provided, as well as a
directory service.
• Maximum packet sizes: Packets from one network may have to be broken into
smaller pieces to be transmitted on another network, a process known as
segmentation or fragmentation. For example, Ethernet imposes a maximum
packet size of 1500 bytes; a maximum packet size of 1000 bytes is common on
X.25 packet-switching networks. A packet that is transmitted on an Ethernet
system and picked up by a router for retransmission on an X.25 network may
have to be fragmented into two smaller ones.