IP Subnet

1. Enter the IPv4 address in dotted-decimal format (e.g., 10.0.5.130) into the IP Address field.
2. Enter either the CIDR prefix length (e.g., /26) or select the subnet mask from the dropdown (e.g., 255.255.255.192).
3. Click **Calculate** to instantly display the subnet mask, network address, broadcast address, first/last host, and total usable hosts.
4. Review the host range to confirm which addresses are valid for devices on that subnet.
5. Use the **Clear** button to reset all fields and calculate a different subnet.

Subnet Mask  = ( 0xFFFFFFFF << (32 − n) ) & 0xFFFFFFFF
Network Addr = IP & Subnet Mask
Broadcast    = Network Addr | (~Subnet Mask & 0xFFFFFFFF)
Total Hosts  = 2^(32 − n)
Usable Hosts = 2^(32 − n) − 2
First Host   = Network Addr + 1
Last Host    = Broadcast − 1

• IP — The 32-bit IPv4 address entered by the user, expressed in dotted-decimal notation (e.g., 192.168.1.45).
• n — The CIDR prefix length — the number of leading 1-bits in the subnet mask (0–32). For example, /24 means n = 24.
• Subnet Mask — A 32-bit mask with n ones followed by (32 − n) zeros, expressed in dotted-decimal (e.g., 255.255.255.0 for /24).
• Network Addr — The base address of the subnet, obtained by zeroing all host bits (bitwise AND of IP and Subnet Mask).
• Broadcast — The last address in the subnet, obtained by setting all host bits to 1 (bitwise OR of Network Addr and the inverted Subnet Mask).
• Total Hosts — The total number of IP addresses in the subnet block, equal to 2 raised to the power of (32 − n).
• Usable Hosts — The number of addresses assignable to devices — Total Hosts minus 2 (network and broadcast addresses are reserved).
• First Host — The lowest assignable IP address in the subnet: Network Address + 1.
• Last Host — The highest assignable IP address in the subnet: Broadcast Address − 1.

1. Subnet Mask: /26 → 11111111.11111111.11111111.11000000 = 255.255.255.192
2. Network Address: 192.168.10.77 AND 255.255.255.192
   → 77 in binary = 01001101; 192 in binary = 11000000
   → AND result = 01000000 = 64
   → Network Address = 192.168.10.64
3. Broadcast Address: 192.168.10.64 OR 0.0.0.63
   → 64 OR 63 = 127 (01000000 OR 00111111 = 01111111)
   → Broadcast = 192.168.10.127
4. First Host = 192.168.10.65
5. Last Host  = 192.168.10.126
6. Usable Hosts = 2^(32−26) − 2 = 2^6 − 2 = 64 − 2 = 62

Result: Subnet Mask: 255.255.255.192 | Network: 192.168.10.64 | Broadcast: 192.168.10.127 | Host Range: 192.168.10.65 – 192.168.10.126 | Usable Hosts: 62

The result tells you that the IP address 192.168.10.77 belongs to the subnet 192.168.10.64/26. This subnet can accommodate 62 usable host addresses (192.168.10.65 through 192.168.10.126). The network address (192.168.10.64) identifies the subnet itself and cannot be assigned to a device; the broadcast address (192.168.10.127) is used to send packets to all hosts on the subnet and is also reserved.

Understanding IPv4 Subnetting

Why Subnetting Exists

IPv4 uses 32-bit addresses, giving roughly 4.3 billion unique addresses. Subnetting divides large IP address blocks into smaller, more manageable segments called subnets. This reduces broadcast traffic, improves security by isolating network segments, and makes efficient use of the limited IPv4 address space.

CIDR Notation

Classless Inter-Domain Routing (CIDR), introduced in RFC 1519 (1993) and refined in RFC 4632, replaced the old Class A/B/C system. A CIDR block is written as an IP address followed by a slash and the prefix length (e.g., 192.168.1.0/24). The prefix length tells you how many bits are the network portion; the rest are host bits.

Subnet Mask Quick Reference

| Prefix | Subnet Mask | Usable Hosts | Typical Use | |--------|-------------|--------------|-------------| | /30 | 255.255.255.252 | 2 | Point-to-point links | | /29 | 255.255.255.248 | 6 | Small office segment | | /28 | 255.255.255.240 | 14 | Small LAN | | /27 | 255.255.255.224 | 30 | Medium LAN | | /26 | 255.255.255.192 | 62 | Medium-large LAN | | /25 | 255.255.255.128 | 126 | Large LAN | | /24 | 255.255.255.0 | 254 | Standard office LAN | | /23 | 255.255.254.0 | 510 | Large campus segment | | /22 | 255.255.252.0 | 1,022 | Large enterprise | | /16 | 255.255.0.0 | 65,534 | Large private network |

Private Address Ranges (RFC 1918)

• 10.0.0.0/8 — 16.7 million addresses, large enterprise networks
• 172.16.0.0/12 — ~1 million addresses, medium networks
• 192.168.0.0/16 — 65,536 addresses, home and small office networks

VLSM (Variable Length Subnet Masking)

VLSM allows network engineers to use different subnet sizes within the same network, allocating exactly as many addresses as each segment needs. For example, a WAN link needs only /30 (2 hosts), while a server farm might need /25 (126 hosts).

Special Addresses

• /32 — A host route representing a single IP address (0 usable hosts in the traditional sense).
• /31 — Used for point-to-point links (RFC 3021); contains 2 addresses, both usable (no network/broadcast convention applies).
• /0 — The default route representing all IP addresses.

• **Confusing the network address with the first host address** — The network address (all host bits = 0) is reserved and cannot be assigned to a device; the first usable host is always network address + 1.
• **Forgetting to subtract 2 for usable hosts** — The formula is 2^(32−n) − 2, not 2^(32−n), because both the network and broadcast addresses are reserved.
• **Using the wrong prefix for /31 and /32** — /31 links (RFC 3021) technically have 2 usable addresses with no broadcast, and /32 is a host route. Treat these as special cases.
• **Entering a host address instead of a network address for routing** — Always use the network address (e.g., 192.168.1.0/24, not 192.168.1.5/24) when configuring routes or ACLs.
• **Mixing up wildcard masks and subnet masks** — Cisco ACLs use wildcard masks (the bitwise inverse of the subnet mask). A /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255.
• **Assuming Class-based boundaries** — Modern networking is fully classless (CIDR). A /20 network is perfectly valid even though the third octet crosses a Class C boundary.

• RFC 4632 – Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan — IETF (Internet Engineering Task Force)
• RFC 1918 – Address Allocation for Private Internets — IETF (Internet Engineering Task Force)
• RFC 3021 – Using 31-Bit Prefixes on IPv4 Point-to-Point Links — IETF (Internet Engineering Task Force)
• RFC 1519 – Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy — IETF (Internet Engineering Task Force)
• IPv4 Addressing and Subnetting – Cisco Documentation — Cisco Systems

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