Router vs Gateway – Why Routers Can’t Convert Different Network Models

1) It is possible to have multiple subnets "behind" one router; the Internet would be a very broken place with millions more routers needed if it were not.

However, your example of using a /16 subnet mask on the LAN interface, with many /24's connecting to it, is not exactly how it would actually function. (More on that in a moment.)

2) There are many use cases. This aggregation and routing of traffic for many networks/ subnets is the purpose of a router.

Take for a simple example, a branch office location with VoIP telephones. At this site, there is a router connected via Dot1q trunking to a layer 2 access switch.

There is, of course, a need for separation of the Voice and Data traffic, which can be achieved by using separate VLANS on the access switches. Each VLAN will have a separate subnet back on our router. Each subnet would terminate on its own "sub interface", a logical partitioning of the physical connection to the access switches.

In this example, there are two subnets (one for data, and one for voice) living "behind" the router.

3) The performance benefit is that you don't need a different physical device to handle the routing for every single subnet in your network. You talk about the traffic ending up on the same router, and it does, but the traffic is still separated unless you specifically allow it to move between the subnets.

The description of switches and bridges is "sort of" correct, "sort of" not.

Bridges typically don't have the capability to filter frames. Switches may have the capability to filter, based on things like access control lists, but that's for a bit later in your networking course.

For right now, consider the following:

A bridge forwards frames from one segment to another segment.

A switch is essentially a multi-port bridge.

To answer your true-or-false questions:

Are the following true or false. Correct them if false:

I can only use either a router or a bridge to connect same protocol following multiple >networks (amongst themselves).

False. You'll typically use switches for this task - in much larger networks, you'll use routers and firewalls between geographically separated sites, but that's beyond the scope of your current studies.

All the devices are gateways falling under different layers of OSI model and have ability >to translate between all the protocols of that layer.(for eg. router is a gateway falling >in network layer and is capable of translating protocols TCP followed by network A to UDP >followed by network B (two of the protocols operating at networking layer) and vice versa)

False. Routers examine Layer 3 packets encapsulated inside Layer 2 frames for network information, and then direct them out interfaces according to their destination. In contrast, a switch looks only at the Layer 2 MAC address to determine its destination. A program (Application Layer) which talks with the UDP protocol will only "talk" on that protocol: the router cannot dynamically change it to "TCP" - it doesn't work that way. The router only encapsulates the data in a format which can traverse links between different Layer 3 networks.

Protocols like TCP and UDP operate at OSI Layer 4 - the "Transport" layer, and they differ in very specific ways and are used for different purposes. For example, UDP has no mechanism to detect whether packets are successfully received at the other end - they might get lost! TCP has a mechanism to detect whether data reaches the other end, and if not, to retransmit the lost packets.

A gateway in general operates in application, session and presentation layer.

False. Gateways operate at Layer 3.

Example: We're going to use a typical home network topology.

Computer A in Los Angeles wants to connect to Website B in New York. Computer A performs a DNS lookup of Website A, which translates to an IP address. Computer A sees that the IP address returned is not on the same network as itself, so it sends the packets to its default gateway. The default gateway is usually a router, which contains a routing table, which tells the router which external networks are reachable through which interfaces - it also has a default route which tells the router which interface or IP address to direct traffic to, if the destination does not appear in its routing table. In this case, because it's a consumer-grade appliance connected to a home network, the gateway sends all packets to unknown destinations upstream to the ISP.