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Subnet Masking

By russellrl2 ·
Throughout the last year we have managed to lose all of our networking personnel. Recently I downloaded a 7-day trial of Fluke Network Inspector. I discovered that most of our switches were subnet masked as 255.255.255.0 while our NT and Unix network are subnetted as 255.255.0.0. We have 2 IP ranges (i.e. 136.128.104.1-255 & 136.128.212.1-255). I was told that we should be using 255.255.255.0. We don't control the router I was a little confused as to why our 1st Cisco switch (connected to the router) did not have a subnet mask. This is the only switch that does not have one assigned. I was told that it was the same as the router since it is our 1st switch. I was also told that it didn't make a difference which subnet mask I used because the router would know where to send. The questions are (1)which subnet mask should I be using; (2) should the 1st switch have the subnet mask assigned; (3) how difficult will it be to change the NT network (6 servers 105 PCs).

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Subnet Masking

by C. Hilbrandt In reply to Subnet Masking

The fact that you don't control the router doesn't matter as your main concern is your LAN. You should be using the 255.255.255.0 subnet mask since you are actually breaking up a class b subnet into a whole bunch of class c subnets.

The subnet mask on your switch doesn't really matter in as far as network connectivity is concerned as long as it is only a OSI layer 3 device (it only makes forwarding decisions by the MAC address, Layer 4 and higher use IP addresses, etc.), which I assume is the case as you have a small LAN and a separate router. The subnet mask is mostly there on a layer 3 switch for SNMP traps and the like.

As far as your 3rd question, hopefully you are using DHCP for handing out your IP addresses. If so, change the subnet mask on the scope and when the PC's renew there addresses, they will get the new subnet mask.
Hope this isn't tooo confusing......

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by russellrl2 In reply to Subnet Masking

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by Craig IT Mangaer In reply to Subnet Masking

Well, if you are connecting to the internet you should definitely be using 255.255.255.0 to ensure that certain sites do not come up as unreachable. Also you could have connectivity issues or delays since your PC with the Class B subnet would think all address are sitting on the same side of the router. So in other words all machines should have the Class C assigned. The switch probably doesn't matter as it simply helps by knowing which IP addresses are on each port and is simply speeding the process of sending traffic to the correct port and cutting down on the broadcast traffic to the rest of the ports. If you aren't Using DHCP for your NT/windows side change to it and make your life simpler. For Unix I won't offer any advice on quick changes as my exposure to that environment has been limited. I can tell you there are two reasons why you have the wrong subnets, one is the way the subnet is set by default corresponding to the original IP address entered. The other possible reason isthat they were trying to make sure the entire network would be visible in network neighborhood(less likely) which a WINS server on each subnet would work much better for maintaining visibility.

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by russellrl2 In reply to Subnet Masking

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by russellrl2 In reply to Subnet Masking

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