Routers

A router is used to route data packets between two networks.
It reads the information in each packet to tell where it is
going. If it is destined for an immediate network it has
access to, it will strip the outer packet (IP packet for
example), readdress the packet to the proper ethernet
address, and transmit it on that network. If it is destined
for another network and must be sent to another router, it
will re-package the outer packet to be received by the next
router and send it to the next router. Routing occurs at the
network layer of the OSI model. They can connect networks
with different architectures such as Token Ring and
Ethernet. Although they can transform information at the
data link level, routers cannot transform information from
one data format such as TCP/IP to another such as IPX/SPX.
Routers do not send broadcast packets or corrupted packets.
If the routing table does not indicate the proper address of
a packet, the packet is discarded. There are two types of
routers:
	

   1. Static routers - Are configured manually and route
data packets based on information in a router table.
   2. Dynamic routers - Use dynamic routing algorithms.
There are two types of algorithms:
          * Distance vector - Based on hop count, and
periodically broadcasts the routing table to other routers
which takes more network bandwidth especially with more
routers. RIP uses distance vectoring. Does not work on WANs
as well as it does on LANs.
          * Link state - Routing tables are broadcast at
startup and then only when they change. The open shortest
path first (OSPF) protocol uses the link state routing
method to configure routes or distance vector algorithm (DVA). 

Common routing protocols include:

    * IS-IS -Intermediate system to intermediate system
which is a routing protocol for the OSI suite of protocols.
    * IPX - Internet Packet Exchange. Used on Netware systems.
    * NLSP - Netware Link Services protocol - Uses OSPF
algorithm and is replacing IPX to provide internet capability.
    * RIP - Routing information protocol uses a distance
vector algorithm. 

There is a device called a brouter which will function
similar to a bridge for network transport protocols that are
not routable, and will function as a router for routable
protocols. It functions at the network and data link layers
of the OSI network model.
Gateways

A gateway can translate information between different
network data formats or network architectures. It can
translate TCP/IP to AppleTalk so computers supporting TCP/IP
can communicate with Apple brand computers. Most gateways
operate at the application layer, but can operate at the
network or session layer of the OSI model. Gateways will
start at the lower level and strip information until it gets
to the required level and repackage the information and work
its way back toward the hardware layer of the OSI model. To
confuse issues, when talking about a router that is used to
interface to another network, the word gateway is often
used. This does not mean the routing machine is a gateway as
defined here, although it could be.

Router is used for communicate two different networks and
use the best path to reach destination.
Gateway: gateway is outer interface of the network.
if you go to one network to another network than all traffic
pass through a gateway.

Router is a layer 3 device,which is used for inter network 
communication and routing purpose but Gateway is a 
entry&exit point of your network.

if u hvae any questions lemme know.