1

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?

3 Answers 3

8

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.

1
  • 3
    I like to think of it this way: when subnetting an existing network, you "throw away" the network you started with.
    – joeqwerty
    Jul 5, 2011 at 23:19
3

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).

0

Yes.

Let's consider /29 which is equal to 255.255.255.248 that is 11111111.11111111.11111111.11111000 in binary representation where 11111111.11111111.11111111.11111 is the network section and 000 is the hosts section.

Therefore the following is possible.

(I've separated the hosts section with a space in the binary representation)

Sub net mask
=============
<-------- network section------><-hosts->
11111111.11111111.11111111.11111   000
Network A available IPs: 192.168.0.0/29
========================================
11000000.10101000.00000000.00000   000 (reserved) - 192.168.0.0
11000000.10101000.00000000.00000   001            - 192.168.0.1
11000000.10101000.00000000.00000   010            - 192.168.0.2
11000000.10101000.00000000.00000   011            - 192.168.0.3
11000000.10101000.00000000.00000   100            - 192.168.0.4
11000000.10101000.00000000.00000   101            - 192.168.0.5
11000000.10101000.00000000.00000   110            - 192.168.0.6
11000000.10101000.00000000.00000   111 (reserved) - 192.168.0.7
Network B available IPs: 192.168.0.8/29
========================================
11000000.10101000.00000000.00001   000 (reserved) - 192.168.0.8
11000000.10101000.00000000.00001   001            - 192.168.0.9
11000000.10101000.00000000.00001   010            - 192.168.0.10
11000000.10101000.00000000.00001   011            - 192.168.0.11
11000000.10101000.00000000.00001   100            - 192.168.0.12
11000000.10101000.00000000.00001   101            - 192.168.0.13
11000000.10101000.00000000.00001   110            - 192.168.0.14
11000000.10101000.00000000.00001   111 (reserved) - 192.168.0.15

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