class _ECDH256: (source)
Elliptic Curve Key Exchange with SHA-256 as HASH. Defined in RFC 5656.
Note that ecdh-sha2-nistp256 takes priority over nistp384 or nistp512. This is the same priority from OpenSSH.
ecdh-sha2-nistp256 is considered preety good cryptography. If you need something better consider using curve25519-sha256.
int giving the preference of the algorithm when negotiating key exchange. Algorithms with lower precedence values are more preferred.