Which of the following are valid IPv4 network masks? (Choose two correct answers.)

255.255.0.0 is a valid network mask because its binary representation (11111111.11111111.00000000.00000000) shows a contiguous sequence of leading ones followed by zeros, representing a /16 prefix.

255.255.255.65 is not a valid network mask because 65 in binary (01000001) contains interspersed ones and zeros, breaking the contiguous block rule for a netmask.

255.255.0.255 is not a valid network mask because its binary representation (11111111.11111111.00000000.11111111) shows a non-contiguous pattern of ones and zeros, specifically a zero octet followed by an all-ones octet.

0.0.0.1 is not a valid network mask because it does not follow the contiguous block of ones followed by zeros rule; it would represent a very small network mask not used in practice.

255.255.255.248 is a valid network mask because its binary representation (11111111.11111111.11111111.11111000) shows a contiguous sequence of leading ones followed by zeros, representing a /29 prefix.