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How much of a WAN static IPv4 /30 subnet is usable ?

Community Veteran
Posts: 5,472
Thanks: 292
Fixes: 4
Registered: 11-08-2007

How much of a WAN static IPv4 /30 subnet is usable ?

I am very lucky to have a legacy static IPv4 /30 (255.255.255.252) subnet on my residential Plusnet internet connection.
According to the Plusnet "Member Centre" the static IPs are allocated as -
    1st  =  "Gateway" address
   2nd  =  "Router" address
   3rd  =  "Spare" address
   4th  =  "Broadcast" address

I am currently using two addresses ("Router" and "Spare"), and am wondering whether I can make use of the others ?
The WAN "Router" address is my default public IP address when connecting via PPPoE to my ADSL modem.
Years ago, the only way I could get the additional "Spare" address to work (on a Linksys router), was was to setup the "Spare" as "IPoA" encapsulation via the WAN "Gateway" address.  Therefore the "Gateway" address serves some purpose.
With my current router, I merely needed to setup a "1:1 NAT" /32 between the WAN "Spare" address and another router on my LAN,  without any reference to the WAN "Gateway" address.  Which implies that the "Gateway" address might not be required.
Is there any reason why I can't (or shouldn't !) do the same "1:1 NAT" /32 routing for the WAN "Gateway" and "Broadcast" addresses ?
From my router's firewall logs, all four WAN addresses appear to be accessible as they all get the same incoming packets being blocked from unwanted external ports scans working through sequential internet addresses.
Is the WAN "Broadcast" address ever used (for broadcast) ?
Will using the WAN "Gateway" address cause problems with the PPPoA configuration of the WAN "Router" address ?
Huh
3 REPLIES
Community Gaffer
Community Gaffer
Posts: 13,284
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Registered: 04-04-2007

Re: How much of a WAN static IPv4 /30 subnet is usable ?

I'll admit I'm not 100% sure of the implications, however with a bit of trickery the broadcast and network addresses can be put to some use.

Bob Pullen
Plusnet Product Team
If I've been helpful then please give thanks ⤵

Community Veteran
Posts: 5,472
Thanks: 292
Fixes: 4
Registered: 11-08-2007

Re: How much of a WAN static IPv4 /30 subnet is usable ?

Thanks Bob !  Smiley
Wow, looks like I'll need to read that description a few times to figure out what was going on there !
The fact that it didn't fail, gives me hope that if I try "1:1 NAT" on the WAN "Broadcast" address using a /32 mask, that it might work.
First I'll go and read "RFC 3021", and find out what on earth "Point2point" is.
Community Veteran
Posts: 5,472
Thanks: 292
Fixes: 4
Registered: 11-08-2007

Re: How much of a WAN static IPv4 /30 subnet is usable ?

For anyone that is interested, here is a link to "RFC 3021" -  "Using 31-Bit Prefixes on IPv4 Point-to-Point Links"