Subnetting

[daveholland]

Hi,

Maybe not a bright question but forgive me my ignorance.

Let's say I wanted to subnet a class B address with subnet 255.255.254.0

so i know i have 126 subnets

510 hosts per subnet

Now here is my question

When i start the first host 2.1 to the last 3.254 that is one subnet


Then I start 4.1 to 5.254 the second one


Now between these subnets do I just use a switch? or a router? Or you just use a router when ur subnet is completely different? Let's say if I had a 255.255.240 address?

[Overlord]

Quick answer - you use a router between subnets.

By the laws of TCP/IP, a subnet of IP addresses (an IP address range) need to go through a router to get to any other subnets, including the Internet

Analogy - a subnet of IP addresses (192.168.1.0/24) is like a room. There is another room next to it, twice as large (192.168.2.0/23). Both rooms which represent IP address subnets are connected by a door into a hallway which interconnects both rooms. The doors are part of that room and use the room's address space.
Real Life - Each IP subnet must have a default gateway IP address from within that subnet to go through a router to reach other subnets.

Challenge - Put 2 subnets on the same physical segment of a network on a switch. No additional VLANs are congigured on the switch.
It works - even though (example) 172.16.1.0/24 hosts are communicating in the same domain as 192.168.1.0/24 hosts, they cannot communicate with each other unless the network traffic goes into the default gateway IP address (router), gets re-encapsulated, then send back out of the other default gateway IP address on the same interface (router) back into the same subnet on the switch.

I recommend referencing how both the layer 2 and layer 3 addressing of a packet look at the following points:
1) leaving the host and going across the the switch into the router
2) leaving the router and going across the switch to the destination host
3) leaving the destination host and going across the switch to the router
4) leaving the router and going across the the switch into the host
Also look at what devices have to use ARP for the MAC address.
H - S - R - S - D
(H = host, S = switch, R = router, D = destination host)