What is IEEE 802.3?

IEEE 802.3 is a working group that wrote the Institute of Electrical and Electronics Engineers (IEEE) standard set , which defines medium access control (MAC) at both the physical and data link layers of wired Ethernet.  This is usually a local area network (LAN) technology with some wide area network (WAN) applications.  Establish physical connections between nodes and infrastructure devices (hubs, switches, routers) through various types of copper or optical cables   

802.3 is a technology that supports IEEE 802.1 network architecture.  802.3 also defines a LAN access method using CSMA/CD. 

What is a Subnet Mask?

A subnet mask is also called a network mask, address mask, or subnetwork mask. It indicates which bits of an IP address identify the subnet of the host and which bits identify the bitmask of the host.  The subnet mask cannot exist alone. It must be used in conjunction with the IP address.  

A subnet mask is a 32-bit address that masks part of an IP address to distinguish the network ID from the host ID, and indicates whether the IP address is on a LAN or a WAN.