Can I announce prefix 161.117.25.0/24 even though I don't have all of /24 IPs? I'm managing one BGP router, under the router I'm trying to announce the following IP prefix:`161.117.25.0/24`. But the problem is that some of the IP addresses were borrowed by other AS(Autonomous System), for example the IP address `161.117.25.100` and `161.117.25.200` were borrowed by other AS. So how should I announce the remaining IP addresses please. Following questions please: 1. Can I announce `161.117.25.0/30`? 2. Is it possible that two Autonomous Systems announce or have the same IP prefix?

That `/24` is part of a larger block (`161.117.0.0/16`) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that `/24`. However, that does _not_ give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.

Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that `/24` used by others would become unreachable.

The conclusion here is: you don't announce the `/24` unless you're authorized to do so.