I feel I understand general subnetting overall, I can answer any questions that the site http://subnettingquestions.com/ gives me but I seem to have a understanding gap some where.

Say an ISP gives me an IP address range to use, ex: - Do they specifically give you address ranges like this? Or would they have given it as -

And if they do give out such a specific range of addresses, and I want to split those up into smaller subnets, and say I want 4 subnets so I do /18 as the mask to get 4 subnets and 16382 hosts per subnet I run into my confusion.

If the address they give out starts at, and if I can't use any address before that like, I would need to adjust my subnet mask so it starts at 10.1 instead of 0.0? But if I use just a /18 I don't see how I can start there.

I understand ANDing the address + the subnet mask, which is why it starts at 0, but how would it start at 0 if I'm given a specific range?

ANDing /18
1 0 1 0 0 1 0 1.0 0 1 1 0 0 0 0.0 0 0 0 1 0 1 0.0 0 0 0 0 0 0 1
1 1 1 1 1 1 1 1.1 1 1 1 1 1 1 1.1 1 0 0 0 0 0 0.0 0 0 0 0 0 0 0
1 0 1 0 0 1 0 1.0 0 1 1 0 0 0 0.0 0 0 0 0 0 0 0.0 0 0 0 0 0 0 0 ANDed result
= starting network id of first subnet, but then that is before the range they gave. (If they give out ranges like I said)

Hopefully I've made some sense here, I've only been studying networking for about a month and half now! Any links or explanations are appreciated!


There is a difference between subnets and ranges.

A subnet is a block of IP space that can be described with a netmask. or is a subnet. TCP/IP routing generally works by looking at the destination IP address and figuring out what entry in the routing table has the most specific enclosing subnet. Routing between different networks over the Internet is done via subnet sizes no smaller than a /24.

A range of IPs is just that, a range. You can start from anywhere and end anywhere. But TCP/IP routing is by subnet. An ISP will generally give customers ranges based on subnets, usually the smallest possible since IPv4 space is scarce.

I doubt an ISP would give you the range - because it would be a pain to set up routing for it, as you have already figured out by trying to create a subnet mask that matches that range. Usually you get a single /24, or if you need more space you get a block of contiguous /24 subnets that can be expressed with a shorter netmask.

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