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Summarising two networks means finding how many high-order bits they share. XOR the two addresses and count the leading zeros: that count is the longest prefix that still contains both, and applying it to the lower address gives the summary network. Fewer common bits means a shorter, broader summary.
Common prefix length
matching bits = count of leading zeros in (A XOR B); summary = (lower address AND mask) / matching bits
A single bit difference high in the address forces a short prefix. Networks such as 10.1.0.0 and 10.254.0.0 only share eight bits, so the only summary containing both is 10.0.0.0/8.
Yes, and that is the point of summarisation — but it also covers everything else in that prefix, so only summarise ranges you own end to end.