OID value: 1.3.6.1.2.1.4.20.1.3
OID description:
ipAdEntNetMask OBJECT-TYPE SYNTAX IpAddress ACCESS read-only STATUS mandatory DESCRIPTION "The subnet mask associated with the IP address of this entry. The value of the mask is an IP address with all the network bits set to 1 and all the hosts bits set to 0." ::= { ipAddrEntry 3 }
See also the OID Repository website reference for 1.3.6.1.2.1.4.20.1.3