non-routable IP address

Looking at RFC 3330, I don't see any IPv4 address reserved for a globally non-routable address.

What I would do is assign -- in local DNS -- an unused local network IP address to the name "blackhole.<domain>" (using the appropriate domain) so you are guaranteed a stable address to which no computer will ever respond. Of course, like always when configuring DNS, ensure that no DHCP server is configured to give out the same address.

According to Wikipedia article on reserved addresses, there are 3 test networks intended for use in documentation only.

192.0.2.0/24
198.51.100.0/24
203.0.113.0/24

Any address in one of those ranges (eg 203.0.113.1) should not map to anything, and indeed should fail to route.

Depending on the type of test you are doing, there are other listed addresses and address ranges that could be appropriate for what you are trying to do.

I think you may want to reword your question. The private address segments (10.0.0.0 - 10.255.255.255, 172.16.0.0 - 172.31.255.255, 192.168.0.0 - 192.168.255.255) are commonly referred to as "non-routable" addresses. I think what you are asking for is a reserved IP address that is reserved specifically to never be assigned. As far as I know there is no address that is reserved in this fashion.

Anything in the

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

ranges will be dropped by any Internet router - they are intended for internal use only and are not routed. There isn't a single address but using one from a block you don't use internally, as long as your router isn't too clever will do the job.

Of course if you have a managed router it may well drop these anyway and not let them go out of the internal network.

RFC-3330 lists these, and lots of other blocks as well