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New Member

squeezing an extra useable address from a /30 subnet

I've got a statically addressed, /30 "public" subnet on a dsl Internet

connection. The usual addressing is in place: a.b.c.0 is the subnet,

a.b.c.1 is the "host", a.b.c.2 is the DSL router's Eth0 and a.b.c.3 is broadcast.

I'm thinking about adding a host configured to use the a.b.c.3 address and

deliberately "mis-configuring" all of the devices with a /29 subnet mask so the new host's address won't be seen as a "broadcast" address. I know this would prevent me from getting to the "a.b.c.4" subnet, but since this is likely to be just another residential DSL user that I'll never need to talk to, I don't much care.

There aren't any routing protocols in use on my subnet. I can't think of anything of consequence that's now doing any layer 3 broadcasting on my "public" subnet.

Based on my firewall logs (logging port scans of sequential addresses) I

know that my ISP is forwarding packets to my "legitimate" broadcast address

(despite their general tendency to block directed broadcasts). I have control over the router (Cisco 675) configuration, all hosts and DNS.

Can anyone think of a reason that this won't work?

1 REPLY
New Member

Re: squeezing an extra useable address from a /30 subnet

That will work because the upstream router (your ISP) will still have /30 on their interface so when it sees a packet from a.b.c.3 it will treat is as a directed broadcast and not forward it on.

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