IP address subnets

So im given a subnet range of x.x.54.128 /27

If your getting this information from your ISP you will also want to confirm with them what the gateway IP is. You will need this to configure your equipment and the gateway IP will not be available to use.  

Typically if an ISP has given me something like x.x.54.128 /27 than x.x.54.129 is configured as the gateway. In that case your usable would be .130-158. Most ISP use the first available for the gateway but I know Comcast uses the last for some reason.

@mlsbraves: the default gateway might be outside of this address range.

It is perfectly legal to have a link with say: 192.168.1.2/24 (where default gw is 192.168.1.1)
And this network range is routed to 192.168.1.2...

@mlsbraves: the default gateway might be outside of this address range.

It is perfectly legal to have a link with say: 192.168.1.2/24 (where default gw is 192.168.1.1)
And this network range is routed to 192.168.1.2...

This is true that the ISP can put the default gateway outside this but in my experience they usually don't. That is why I suggest if this is from an ISP that the Question Author contact their ISP to confirm. Call and say you received the circuit info but want to confirm the default gateway address and usable IP. That's the best way to start documenting the IP that are going to be used.

Typically when I order say a /29 they know I wan't 5 usable IPs. So I normally see: x.x.0.1 /29
Where x.x.0.1 is the default gateway and 2-6 are my usable.

Then again Comcast usually uses the last usable x.x.0.6 and my usable are 1-5.

Just depends on ISP so contact them if this IP info was sent from an ISP to confirm everything.

192.168.1.2/24 (where default gw is 192.168.1.1)

A /24 would take up the entire last ocet in that example. 192.168.1.0 /24 (192.168.1.1-254)
Maybe 192.168.1.1 /30 Default Gateway and 192.168.1.4 / 25 would work.

The customer could create a manual route for anything outside the subnet but I've never seen it done that way. Typically the default gateway needs to be inside the subnet. In order to talk to a device outside its subnet it needs to contact the gateway which is accessible within its own subnet.

Thank you.