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I have a 192.168.1.64 /28 Network

Can I divde this network into

192.168.1.64/29 

and

192.168.1.72/29

?

My Question is related to the fact that now the "network address" (192.168.1.64) of the /28 is shared also by the 192.168.1.64/29 network.

And the Broadcast address (192.168.1.79) of the /28 network is now shared by the 192.168.1.72/29 network.

Is this allowed? Or do I have to, if I split the /28, make sure that the network/broadcast addresses do not overlap? Do I have to use a /30 if I subdivide the /28 network?

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2 Answers 2

up vote 7 down vote accepted

Yes you can, easily - the 'network address' is something of a misnomer in this situation as you'll kind of have two, what you really need to care about is the fact that you'll now need two default gateways, one per /29.

Essentially what you'll have is;

  • 192.168.1.64 - 'network address' of first /29 subnet - could theoretically be used as default gateway.
  • 192.168.1.65 - likely real DG for first /29 subnet
  • 192.168.1.66 - server IP #1
  • 192.168.1.67 - server IP #2
  • 192.168.1.68 - server IP #3
  • 192.168.1.69 - server IP #4
  • 192.168.1.70 - server IP #5
  • 192.168.1.71 - broadcast address for first /29 subnet

  • 192.168.1.72 - 'network address' of second /29 subnet - could theoretically be used as default gateway.

  • 192.168.1.73 - likely real DG for second /29 subnet
  • 192.168.1.74 - server IP #1
  • 192.168.1.75 - server IP #2
  • 192.168.1.76 - server IP #3
  • 192.168.1.77 - server IP #4
  • 192.168.1.78 - server IP #5
  • 192.168.1.79 - broadcast address for second /29 subnet

Hope this helps, oh and I'm sure you know but a /29 works out as 255.255.255.248.

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2  
I like to think of it this way: when subnetting an existing network, you "throw away" the network you started with. –  joeqwerty Jul 5 '11 at 23:19

Yes, you can.

# ipcalc -mnb 192.168.1.64/28
NETMASK=255.255.255.240
BROADCAST=192.168.1.79
NETWORK=192.168.1.64

# ipcalc -mnb 192.168.1.64/29
NETMASK=255.255.255.248
BROADCAST=192.168.1.71
NETWORK=192.168.1.64

# ipcalc -mnb 192.168.1.72/29
NETMASK=255.255.255.248
BROADCAST=192.168.1.79
NETWORK=192.168.1.72

The overlapping you mention should be of no concern, since for most purposes you won't be using 192.168.1.64/28 (except for aggregation in routing, which would be fine).

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