🌐 IP Subnet Calculator

Subnetting is a technique to divide IP networks into smaller logical units. CIDR (Classless Inter-Domain Routing) notation displays IP addresses followed by a slash (/) and the number of network bits. For example, 192.168.1.0/24 means the first 24 bits are the network portion. Subnet masks distinguish network and host portions, with /24 equivalent to 255.255.255.0. Classful addressing (A, B, C) was inefficient and replaced by CIDR. /32 represents a single host, /31 provides 2 addresses for point-to-point links. Subnetting enables efficient network management and enhanced security.

Network address is the first address in a subnet with all host bits set to 0. The network address of 192.168.1.0/24 is 192.168.1.0. Broadcast address is the last address with all host bits set to 1, becoming 192.168.1.255. Network and broadcast addresses cannot be assigned to hosts. Usable host addresses equal total addresses minus 2. /24 provides 254 usable addresses from 256 total. /30 offers only 2 usable addresses from 4, suitable for point-to-point links. IPv6 uses multicast instead of broadcast.

Subnet masks consist of consecutive 1s and 0s in binary. /24 is 11111111.11111111.11111111.00000000 or 255.255.255.0. /28 has 28 bits of 1s, resulting in 255.255.255.240. To convert CIDR to subnet mask, add (32 - CIDR) bits of 0s. Conversely, counting 1 bits in the subnet mask gives the CIDR value. VLSM (Variable Length Subnet Mask) allows subnets of varying sizes. Wildcard masks are inverted subnet masks used in ACL configurations. Supernetting combines multiple networks into one.

Let's examine dividing a corporate network by department. A company has 10.0.0.0/8 network. Sales team needing 500 people fits /23 (510 hosts). Development team of 100 suits /25 (126 hosts). Small branches use /28 (14 hosts). Server-to-server connections employ /30 (2 hosts). Subnet planning should consider future growth and reserve extra addresses. Hierarchical addressing simplifies routing tables. Grouping address blocks by region and function eases management. Documentation tracks IP address allocation status.

Routers forward packets between different subnets. Hosts within the same subnet communicate directly, while different subnets require routers. Routing tables contain destination networks and next-hop information. Longest prefix matching selects the most specific route. CIDR aggregation summarizes multiple subnets into one route. 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24 can aggregate to 10.1.0.0/22. Default route (0.0.0.0/0) handles all unknown destinations. Inter-subnet communication is controlled by firewalls to enhance security.

IP subnet calculators are essential for network design and troubleshooting. Entering IP and CIDR instantly calculates network address, broadcast, and usable hosts. Appropriate CIDR can be reverse-calculated from required host count. Needing 100 hosts means 2^7=128, so /25 is suitable. Subnet division follows powers of 2. Dividing /24 into 4 parts yields /26. Avoid overlaps to prevent IP conflicts. Use private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) for internal networks. Online tools verify and visualize complex VLSM plans.