Defualt Gateways

flares2

Hey guys,
Forgive me if this is a stupid question (but like they say, there's no stupid questions, only stupid people who ask them.)
Something's been getting to me about default gateways. Does each subnet require a different router interface for its default gateway? For example if I'm using a /29 subnet (62 subnets with 2 hosts each), do I need 62 seperate router interfaces? And if so, the default gateway for each subnet would be one of the available IPs, so thats only leaves one IP per subnet. I can't imagine using 62 router interfaces for 62 workstations (though this would eliminate collisions).
I read that you can assign multiple IPs to a single interface, but doesn't this negate the idea of seperating collision domains through subnetting because multiple subnets would still be going through a single port?
Also, I haven't gotten too into Layer 3 switching yet. So, does Layer 3 Switching play a part in this?

Thanks guys,
Dave

PawNtheSandman

You can use 1 default gateway for them all. If you have 2 ways out of your network you can split them, etc.

Default gateway is the computer ip that the internal users use to reach outside your network.

BubbaJ

Subnets with a 29 bit mask can have 6 hosts per subnet - not just 2. A 30 bit mask will give you 2, and is generally used for point-to-point connections. Note: Cisco supports 31 bit subnet masks for point-to-point connections.

In general, you do need a router address per subnet. This assumes that you want the hosts to be able to access devices on other networks. You can accomplish this with trunking and subinterfaces.

Collision domains are separated by a layer 2 device such as a bridge or switch. Routers separate broadcast domains (VLANs).

A switch with multiple subnets connected to it should have a different VLAN per subnet. Each switch port is a different collision domain. Modern practice is to use one device per switch port, and, if using full duplex, there will be no collisions.

The switch will combine the traffic streams to one or more trunks, but only if it needs to. If a host is sending to another device on the same VLAN, the traffic will not be sent on the trunk to the router.

Danman32

Although I have seen working exceptions to this (that really should not have worked), a default gateway IP on a host would be the IP of the router interface that is on the same subnet as the host. So for each subnet, all the single interface hosts would share the same default gateway, but but would be different from subnet to subnet. In the case of the router that is connecting all these subnets together, there would be one default gateway on the router pointing to the next step out from the router for any network that the router doesn't otherwise know about, which in most cases would be the way out to the internet. The router's default gateway IP would be the IP of the next router's interface that is on the same segment as one of the interfaces of this router.

Think of a house with many rooms, each room with many doors. Each room would have one door designated as the next step to get out of the house. The next step may have you end up in another room, but if the proper doors are chosen as the 'default door', you'll be one step closer to getting out of the house. The house itself could be thought of as a router, and it has only one door, the default gateway, that leads to the outside.