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Why do routers have only a few interfaces? Since one router interface is one subnet itself, a router given G0/0, G0/1 and G0/2 can only have three subnets.

Is it sustainable? How can we go beyond three subnets, or is this where VLAN plays a part?

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    What do you mean by "have three subnets" exactly? What does it mean for a router to "have" a subnet? – David Schwartz Oct 5 '17 at 17:50
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    L3 switches are also routers and they can have several hundred ports... – Zac67 Oct 5 '17 at 17:59
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    <shrug> Not all routers are created equal. A top-of-the-line Cisco CRS-3 multishelf platform apparently supports up to 1152 slots. Assuming 1000 of these are usable for linecards and linecards have up to 10 ports each, there's 10000 physical ports, even before you start creating virtual interfaces. Even a single shelf should easily give you over 100 physical ports. – Digital Trauma Oct 5 '17 at 18:11
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    Who says you can't put more than one subnet per physical interface? I've done it for close to 30 years. – Ricky Beam Oct 5 '17 at 21:31
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 19:25
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You forget that routers can have virtual interfaces. For example, you can create GigbitEthernet0/0.10. You can have many, many different VLANs, each with its own network, on virtual interfaces of a single physical interface.

Routers only need to terminate the layer-2 LANs. It is the LANs that really need a lot of interfaces, so you use switches for the LAN interfaces, and the switch connects to the router (with a trunk if you have more than one VLAN).

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This is 3 question in your post, so I'll try to answer the 3

1 - there is routers with loooooots of interfaces. most switches are layer 3 switches, which is a router. Lots of enterprise routers only use 2 or 3 ports just because most enterprise don't need more. In this case the router has an "outside" interface (connected to the Internet) and an "inside" interface, which is typically connected to a layer 2 switch.

2 - You can have multiple subnets on 1 interface with secondary ip address (in Cisco language)

Interface Ge0/0
ip address 1.1.1.1 255.255.255.0
ip address 2.2.2.1 255.255.255.0 secondary
ip address 3.3.3.1 255.255.255.0 secondary

Note that if 1.1.1.2/24 want to communicate with 2.2.2.2/24 all trafic will go through the router. This has nothing to do with VLAN... It's just that 2.2.2.2/24 is outside of 1.1.1.0/24 so all trafic has to be send to the router/gateway (1.1.1.1)

3 - VLAN allow you to create virtual interface on the router using an identifier (the VLAN). Layer 2 trafic is impossible between 2 vlan (it's like 2 distinct network) but the router will be able to route trafic from 1 vlan to another

Interface Ge0/0.10
encapsulation dot1q 10
ip address 1.1.1.1 255.255.255.0

Interface Ge0/0.20
encapsulation dot1q 20
ip address 2.2.2.1 255.255.255.0

Of course, Vlan needs to be configured at the switch level.

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