What is the function of the SUBNET MASK in IPv4?

1. An IPv4 address is divided into NETWORK and HOST portions. The subnet mask says where the division is. 2. Example: address 192.168.1.42 with mask 255.255.255.0 (or /24): - The first 24 bits (192.168.1) is the NETWORK identifier. - The last 8 bits (42) is the HOST within that network. 3. The mask consists of $1$-bits for the network portion, $0$-bits for the host portion (always contiguous). 4. CIDR notation /24 means 24 network bits. Common masks: /16, /24, /28, /30 for point-to-point links. 5. Other options misstate the role. _Source: RFC 1180 "A TCP/IP Tutorial", §5 (IP — Addressing and Subnetting)._