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Lets say I own (LIR-status) the prefix 80.200.72.0/21 and that prefix is routed via AS e.g. AS12345 at this very moment.

Now I want to split this prefix into two AS, so the last portion (80.200.79.0/24) of the prefix mentioned before is routed via a new AS e.g. AS54321 and the rest is routed via the existing AS AS12345.

Is this possible?

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Yes, no problem at all, that's what BGP is for. You can do this by announcing the /24 from AS54321 and announcing the remaining /22, /23 and /24 from AS12345, or announce the /24 from AS54321 and the /21 from AS12345. The last option will work since more specific routes are preferred.

If you want your prefixes to be globally visible, don't use anything smaller than a /24. Also, you may need to fix proper routing object in routing registries like the RIPE database and announce to your upstreams that you'll be advertising the prefixes, so they can update their filters and will accept the announcement.

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