Subneting

DerekAustin26

1.) Why is it regardless whether you have a Private or Public IP address the subnet mask is always "255.255.255.0" when you do an ipconfig on Class C IP's in your command prompt? Its never like what the book talks about.. like 255.255.255.128 or .192 or 224,240,248,252 ? Is this just something the ISP sees, the router has? I dont understand.. Could you elaborate on that? (Keep in mind i know exactly what a Private IP is and NAT addressing and their purpose, but these questions are based specifically on Public IP's).......

2.) Easy question.. When using Public IP's with subnets of 255.255.255.0 does this mean the user NEEDS 254 hosts? Like if he only needed 126 hosts, wouldnt the ISP give him a subnet mask of 255.255.255.128? (Keep in mind i know exactly what a Private IP is and NAT addressing and their purpose, but these questions are based specifically on Public IP's)......

3.) This may be redundant but i really wanna make sure I got this down... So, instead of ISP's just wasting all their ips when using Public IP's wouldnt they just use the subnet mask 255.255.255.252 for MOST Home users since most Home users only have about 1-2 pc's per household?

miller811

Typically in a home network you are not concerned with reserving any address space. so the standard seems to be 255.255.255.0 allowing for 254 hosts on that subnet.

You could easily configure you how network to be .192. scheme etc. if you wanted to.
If you did configure you home network with .192 scheme you would need to know subnetting to make sure the hosts are in the same part of the subnet

192.168.1.0 255.255.255.192 would have the following subnets
192.168.1.0 subnet valid hosts 192.168.1.1-192.168.1.62 broadcast of .63
192.168.1.64 subnet, valid hosts 192.168.1.65-192.168.1.126, broadcast of .127
192.168.1.128 subnet, valid hosts 192.168.1.129-192.168.1.190, broadcast of.191
192.168.1.192 subnet, valid hosts 192.168.1.193.-192.168.1.254, broadcast of .255

Unless you know how to subnet, which the average home user would not know how to, it would be to confusing to use the proper addressing range.

Your ISP is typically assigning you a single IP address, not a range of 254 addresses.
here is my info for example off of my router

IP Address 71.228.2xx.XXX
DHCP DHCPClient
IP Subnet Mask 255.255.252.0

The ISP has handed me single address which my home router when set to a 255.255.255.0 scheme will NAT up to 254 host to that single address.

When you run IP config on your home pc and look at the output, it has nothing to do with your public facing address ISP has handed you. The output you are seeing is the scheme your home routers is using.

A common area you would see subnetting used is on point to point wan link with a .252 subnet which only allows for 2 valid host addresses, 1 for each router on the point to point link.

hope that helps.