Network Devices: Part 3: Distribution and core level devices

Router

A router is a networking device that forwards data packets between computer networks.

The main purpose of a router is to connect multiple networks and forward packets destined either for its own networks or other networks. A router is considered a layer-3 device because its primary forwarding decision is based on the information in the layer-3 IP packet, specifically the destination IP address.

How router works?

• When a packet is coming on interface (ingress), the router needs to know which interface it has to use to send out the packet (egress). Router checks this against the routing table which has entries related to each known destination. Each routing entry stores destination address and address of next device on the path called next hop device.
• Now, having next hop address, router can determine the outgoing interface (egress) and send the packet. The same thing happens on the Router 2 and the data reachs the destination address

Layer 3 switch combines the functionality of a switch and a router. It acts as a switch to connect devices that are on the same subnet or virtual LAN at lightning speeds and has IP routing intelligence built into it to double up as a router. It can support routing protocols, inspect incoming packets, and can even make routing decisions based on the source and destination addresses.

Features of router:

• Support routing between virtual LANs
• Improve fault isolation
• Security management
• Separate routing tables, and as a result, segregate traffic better
• Support flow accounting and high-speed scalability
• Lower network latency as a packet doesn’t have to make extra hops to go through a router

But on another hand:

• Cost
• Complex configuring and administering
• Slower than L2 switches
• Mostly applicable only for large intranet environments with many device subnets and traffic

Gateway

Gateway is a hardware router or software for interfacing computer networks using different protocols (for example, local and global). Gateways are distinct from routers or switches in that they communicate using more than one protocol to connect a bunch of networks and can operate at any of the seven layers of the OSI model. Usually firewalls - traffic filtering software - installed on gateways to provide network security.

Modem

A modem (modulator-demodulator) – is a hardware device that converts data into a format suitable for a transmission medium so that it can be transmitted from one computer to another (historically along telephone wires). Modems can be used with almost any means of transmitting analog signals from light-emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data.