It is worth mentioning that neither in IPv4 nor in IPv6 is there a limitation on announcing prefixes. We could say that in IPv4 they do it this way to avoid overpopulation of routing tables and to not allow prefixes from internal networks to be seen on the Internet, in this way operators usually filter prefixes greater than /24 in IPv4.

Similarly, in IPv6 the RIRs do not usually assign greater than /48, although this prefix length is not fixed, it has varied over time and may vary in the future.

In the context of BGP (Border Gateway Protocol) and common ISP (Internet Service Provider) practices, the advertised prefix size may vary depending on ISP policies and recommendations from regional Internet registries (RIRs such as ARIN, RIPE, APNIC, etc.).

However, there are de facto industry standards for the minimum prefix size that is advertised over BGP to optimize the scalability of routing tables on the Internet.

For IPv4

The minimum prefix commonly accepted and advertised by ISPs in BGP is final match.. This is the smallest block that most ISPs will advertise or accept due to filtering policies designed to keep the size of the global routing table manageable. Advertising smaller blocks (such as /25 or /26) could result in these ads being filtered and not propagated across the global internet.

For IPv6

In the case of IPv6, the minimum prefix typically advertised by ISPs is final match., although /32 prefixes are common for assignments to clients. Internet registries often allocate /32 blocks to ISPs, which then subdivide these blocks to allocate to their customers.

Some ISPs may choose to advertise larger blocks (such as /32 or /36) depending on their address allocation policy and the structure of their network, but /48 is generally the minimum accepted prefix size for most IPv6 routes in Internet.

Implementation in MikroTik

On MikroTik routers, the BGP configuration allows you to specify the size of the prefix to advertise. The decision on the size of the prefix to advertise should be based on industry best practices and the policies of upstream providers and peers.

To advertise a prefix on MikroTik via BGP, you will need to configure the appropriate parameters in the RouterOS BGP menu, including defining your BGP networks and peers.

It is important to note that although you can technically configure your MikroTik router to advertise any prefix size, following the best practices and acceptance policies of your peers is crucial to ensuring that your advertisements are accepted and propagated effectively across the Internet.