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Reducing a /16 network to a /24

Posted on 2016-08-09
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I have a live network that is in the 10.10.0.0/16 range.  I am creating VLANS and subnets and moving most of the nodes out of the range all together.  I was wondering though, is there a way to create a vlan/subnet in the 10.10.1.0/24 range and keep communications up until I can change the grandfathered network to the /24 range?  Right now I'm going to have to take the network down, change the masks change the grandfathered range to a 24 and then create the VLAN that I need.

Anyway to do this with a live network?  Working with a Cisco 6509...
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Question by:publicvoid
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SIM50 earned 500 total points
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I don't see how this would work. Longer prefix will be put in the routing table thus cutting off traffic flow to /16.
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by:publicvoid
ID: 41749470
Thought so but I though maybe there was some ninja solution I didn't know about.
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by:Bing CISM / CISSP
ID: 41749767
> a way to create a vlan/subnet in the 10.10.1.0/24 range and keep communications up

there is NO problem for hosts with /16 mask to communicate with others with /24 mask, IF:

1. they are in the same phsicsl network, VLAN or broadcast domain

AND

2. the /16 mask hosts are actually in the range of 10.10.1.0/24.

e.g. 10.10.1.100/16 can talk with. 10.10.1.200/24, but 10.10.2.100 cannot talk to 10.10.1.100/24 (technically the latter can receive the former's packets but will reply to other subsets if a default gateway is available).

does it make sense?

therefore if you carefully plan the IPs, new hosts can communicate with SOME hosts on the grandfathered network, though not all of them.
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