[Gllug] Best practice: Multiple sub-netting/NAT

[Anthony Newman]

DL Neil wrote:
> Sorry Bruce, but we're not all Network Admins/experts - and hence need  
> to ask questions to learn... Please feel free to correct my use of  
> terminology.

Keep your hair on. The best way to get free advice is to ask nicely ;)


> The question was not about sub-net numbering, but since you ask  
> 192.168.0.0 and 192.168.1.0.

It is useful to have these sort of details when answering questions 
unless they're s00per sekrit for some reason, as it makes problems 
easier to envisage.


> The first question was whether the two sub-nets could be combined into  
> one. 

If you don't need to partition your network (e.g. for firewalling or 
other routing reasons) and you have nedd for less than 254 addresses, 
you'd be better off consolidating I'd have thought.

If you presently have a router of some description connecting the two 
networks, it will have to go if you wish to combine the two networks 
effectively into one large segment.


> Thereafter I assume the sub-net MASK would have to change (from  
> 256.256.256.0) because addresses which are masked are on the LAN (and  
> thus don't go out throught the router) whereas those which are not  
> should be routed.

255, not 256 - this is the largest positive number you can represent 
with 8 bits, which is one quarter of the 4 byte IPv4 address. The subnet 
mask represents the bits which do not change within a subnet, which for 
255.255.255.0 (i.e. a /24 denoting 24 bits of fixed address prefix) 
would represent 8 bits or 255 addresses worth of available space.

The next larger netmask is 255.255.254.0 ( /23 - note the final *9* bits 
are now 0) containing 511 addresses. Not all of these are necessarily 
usable if you require a broadcast address at the end, and the first 
address is reserved.


Ant
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