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Subnet Mask same for all hosts on network?

post #1 of 5
Thread Starter 
As a component of my high-school level "Introduction to Networking" (TEJ4M) course, I've been assigned task of designing a small-business network for a fictitious company. Not sure how to calculate subnet mask for each IP address however.

I've sub-netted the Class B network for 120 hosts in 8 separate departments, with their own sub-networks and IP addresses, but unsure of whether or not the sub-net mask remains the same throughout the network.

Borrowed 4 network bits, increment is 16 and mask is 240. So, would the subnet mask for the entire network be 255.255.240.0?

11111111.11111111.11110000.00000000

Or would the subnet mask for host 1 from the payroll department (for example), have a different subnet mask than that of host 5 from the R&D department?

Example IP Addresses (small sample):
Payroll: 128.0.16.1
128.0.16.2
128.0.16.3

R&D: 128.0.48.1
128.0.48.2

I've only received a basic introduction to subnetting, and am competent in binary counting.

Thank you!
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post #2 of 5
Best practice is to make 1 subnet = 1 broadcast domain to keep things simple. So each subnet gets its own 255.255.255.0 netmask, and broadcast address of x.x.x.255.

You can think of each subnet as the smallest possible unit that can be serviced by a single router. Can you have 128.0.16.1 on router A and 128.0.16.254 on router B? Nope, because then a broadcast from 128.0.16.254 will not reach 128.0.16.1. The netmask for the entire network is, as you said, 255.255.240.0 (this includes 16 subnets of 254 hosts each).

A lot of protocols (such as DHCP) make the 1:1 assumption. In practice, there are many ways to work around this and do 1 subnet != 1 broadcast domain, yet still get your network to work perfectly. However, other people might have difficulty understanding your network.
Edited by thefreeaccount - 2/17/11 at 3:49am
post #3 of 5
The mask itself depends on the subnet.
If you wanted to divvy up something like 192.168.1.0/24 into two networks, you'd have to borrow another bit when using the subnet mask. This would give you 192.168.1.0/25 as well as 192.168.1.128/25 as separate networks. With /25 your network mask would now be 255.255.255.128, instead of 255.255.255.0 when it was a /24 network.

This is a decent read if you're just getting into networking/subnetting:
http://www.techrepublic.com/article/ip-subnetting-made-easy/6089187
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post #4 of 5
If there are 120 hosts in each subnet, then each network is /25 or netmask of 255.255.255.128

Why are the subnets so much larger than what is needed?

Also, the "main distribution center"? Is that another router? Did you include a network subnet for the routers? Or is that not your drawing?
post #5 of 5
Thread Starter 
Quote:
Originally Posted by thefreeaccount;12417792 
Best practice is to make 1 subnet = 1 broadcast domain to keep things simple. So each subnet gets its own 255.255.255.0 netmask, and broadcast address of x.x.x.255.
Thank you! I appreciate your help.
Quote:
Originally Posted by herkalurk12419281 
Why are the subnets so much larger than what is needed? Also, the "main distribution center"? Is that another router?
The main distribution centre refers to the server closet. The instructor asked us to ignore everything having to do with the server hardware, and focus predominantly on networking components.
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