Summarize CIDR Blocks to Supernet

Your ACL has forty /24 lines in it and the temptation is to collapse them into one. The question sounds harmless, but summarizing blocks is precisely where firewall rules quietly grow too wide. The new line goes in, traffic keeps flowing, and riding along inside it are ranges that were never on the original list.

This box does one specific thing, and it is worth knowing which. It takes the lowest address and the highest address across the whole list and returns the smallest prefix covering both ends. That is not route aggregation: it does not merge only adjacent blocks and it does not return a minimal list. From 10.0.0.0/24 up to 10.0.3.0/24 you get a clean 10.0.0.0/22. But 10.0.0.0/24 with 10.255.255.0/24 becomes 10.0.0.0/8, over 16 million addresses.

Always read the address count line before moving a result into a rule. If the total exploded, your blocks are not contiguous and the honest answer is to keep several lines. Mixing 192.168.1.0/24 with 10.0.0.0/8 returns 0.0.0.0/0, which is the entire internet. Every entry needs its prefix, host bits are masked off so 203.0.113.7/24 becomes 203.0.113.0/24, and only IPv4 is accepted, one per line or comma separated.