Many others have already recommended these, but I suggest installing test VMs of both phpipam and nipap and seeing which works best for your use case.
NIPAP has fairly extensive tools supporting automation for provisioning. phpipam has a few additional functions on top of only ip address management, it also appears to have been designed for a use case where people are running geographically spread out layer 2 services and keeping track of which vlan belongs to which customer.
I tried all 3 netbox, nipap and phpipam as a gui user and I like the phpipam the best as it has most knobs and features and is very intuitive, (can't comment on the api richness, yet)
But what is strange is that none of these tools provides RT (or extended/standard communities or even vlan for that matter) management the way they provide IP management, that is allow one to create custom pools and then have the next free resource allocated from a pool if needed.
adam
You might also want to look at 6connect.
Owen
From: Owen DeLong <owen@delong.com>
Sent: Friday, September 6, 2019 4:52 PMYou might also want to look at 6connect.
I actually did recently, to potentially migrate from phpipam, but I couldn't
even drive the IPAM thing 
Just saying, in phpipam I didn't have any trouble figuring out basic stuff
like how to name a subnet, assign IP to A-side host and B-side host on a p2p
link with /31 (or /30) subnet, or how to create children off of a parent
block in no time.
Though what I like about 6conenc is that its insanely customizable and I
like the automatic chopping of subnet to equal sized blocks and ability to
merge etc., but... as I said couldn't drive even the basic stuff...
And regarding the automation capabilities -I actually like the full blown
ONAP style automation better.
adam