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Subnet Question

Posted on 2003-12-06
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Last Modified: 2011-09-20
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My network currently uses a Cisco PIX 515E with an internal layer 3 switch and we are going to be implementing NAT on the PIX. Our inside network will be 192.168.0.0 that will translate to a pool consisting of 5-6 class C networks.

I would like to use more than 1 subnet mask to chunk up the 192.168.0.0

255.255.255.0 will be used for my smaller vlans and I would also like to use 255.255.248.0 for two of my large vlans.

So my question is, Can I use both subnet masks as long as they do not overlap?
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Question by:mikesparker
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ShineOn earned 25 total points
ID: 9891456
192.168.x.x is class C.  The 255.255.248.0 mask is a class B subnet.  You might want to do supernetting, where using CIDR subnetting, you can aggregate networks.

How big is your network in number of nodes, number of sites, and number of vlans you want to establish?

You may want to use a class B network or even a class A network, and subnet it according to its class.

VLANs, IIRC, are supposed to be classless - in other words, a VLAN is not equivalent to a subnet.

I'm sure some other Experts will chime in with either contradictory statements or more detailed explanations.  This is just to get you started...
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Expert Comment

by:chicagoan
ID: 9891618
192.168.0.0 255.255.248.0 would yield you up to 32 subnets in 192.168.0.0/16 with 2048 addresses including network and broadcast
i.e.
network         from                to                broadcast
192.168.0.0 192.168.0.1 192.168.7.254 192.168.7.255
etc. Note your want to exclude .0 and .255 node addresses within that range to accomodate windoze

the 255.255.255.0 will obviosly net you 256 subnets of 256 addresses

>Can I use both subnet masks as long as they do not overlap?
If your routing protocols understand VLSM, yes

using 10.0.0.0 255.255.0.0 might simplify things, help desks and techs sometimes have a hard time getting their head around vlms's and having a consistent mask throughout the organization has it's merits from the standpoint of simplicity.







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Author Comment

by:mikesparker
ID: 9892061
I was considering the following setup. It's for a small University with about 1000-1200 machines. I will be using a Catalyst 6513 with Sup2/MSFC2. So will this work? Do I just enable ip classles in the MSFC2?

VLAN1:  PIX to 6513            192.168.1.0 255.255.255.0

VLAN2: Servers                  192.168.2.0 255.255.255.0

VLAN3: Printers                  192.168.3.0 255.25.255.0

VLAN4: Switch/Router         192.168.4.0 255.255.255.0
           Management

DMZ:                                 192.168.5.0 255.255.255.0

Faculty/Staff                      192.168.16.0 255.255.248.0

Student Labs                      192.168.20.0 255.255.248.0
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by:chicagoan
chicagoan earned 100 total points
ID: 9892081
as long as you're not using RIP ;)

Is this thing completely flat?
No routers outside of the cat?
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Author Comment

by:mikesparker
ID: 9892107
Thanks. Yes, we just have 1 L3 switch (and many L2 switches). I think I will use EIGRP.
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Expert Comment

by:chicagoan
ID: 9892124
EIGRP or OSPF will do fine

You're creating some fairly large VLANs, I'd consider breaking things down at your L2 switch levels for comparmentalization of problems, but I don't know the history or purpose of the idea, so buona fortuna!
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Author Comment

by:mikesparker
ID: 9892496
I agree. However, right now we have 1 VLAN for EVERYTHING... hehe. I am new on the job and my jaw hit the floor when I saw the campus setup. I think 7-8 VLANS should eliminate our broadcast storms. I'm a bit nervous though because I am going to implement DHCP, NAT, EIGRP (currently RIP), and the VLANs all at the same time. We are just waiting for the new 6513 to arrive...
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Expert Comment

by:chicagoan
ID: 9892497
sounds like fun

just remember:
You're NEVER done!
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