Key Features
- IPv4 & IPv6 Support – Calculate subnets for both address types
- CIDR Notation – Convert between CIDR and subnet mask formats
- Network Details – View network address, broadcast, first/last IP
- Wildcard Mask – Calculate Cisco wildcard masks automatically
- Host Calculations – Determine usable host addresses per subnet
- Binary Conversion – View IP addresses in binary format
- Subnet Division – Split networks into multiple subnets
- VLSM Support – Variable Length Subnet Masking calculations
- Quick Reference – Common subnet mask table
- Browser-Based – No installation required
Understanding CIDR Notation
CIDR (Classless Inter-Domain Routing) notation is a compact method to specify IP addresses and their associated network masks. It uses a slash followed by a number to indicate how many bits are used for the network portion. CIDR was introduced to replace the older classful addressing system (Class A, B, C) and improve IP address allocation efficiency.
The CIDR number represents consecutive ones in the binary subnet mask. For example, 255.255.255.0 in binary is 11111111.11111111.11111111.00000000, which contains 24 consecutive ones, expressed as /24.
/24 Network
CIDR: 192.168.1.0/24
Subnet Mask: 255.255.255.0
Usable Hosts: 254
Most common for small LANs. Provides 254 usable IP addresses with network and broadcast addresses reserved.
/16 Network
CIDR: 10.0.0.0/16
Subnet Mask: 255.255.0.0
Usable Hosts: 65,534
Ideal for medium to large organizations. Allows for extensive network segmentation and growth.
/30 Network
CIDR: 192.168.1.0/30
Subnet Mask: 255.255.255.252
Usable Hosts: 2
Perfect for point-to-point links between routers. Minimizes IP address waste on network links.
Common Use Cases
Network Planning
Design network topology, plan IP address allocation, determine subnet requirements for departments or VLANs, and optimize address space utilization.
Router Configuration
Configure routers and switches with correct subnet masks, set up routing tables, implement access control lists (ACLs), and calculate wildcard masks for Cisco devices.
Security Segmentation
Isolate sensitive systems on separate subnets, implement network security zones, control traffic between departments, and improve overall network security posture.
IPv6 Migration
Plan IPv6 address allocation strategy, understand IPv6 prefix delegation, calculate subnet sizes for dual-stack environments, and prepare for future network growth.
IPv4 vs IPv6 Subnetting
While IPv4 and IPv6 both use CIDR notation, they differ significantly in address space and subnetting practices.
IPv4 Subnetting
- 32-bit addresses: 4.3 billion possible addresses
- Common prefixes: /8, /16, /24, /30
- Conservation critical: Address exhaustion concerns
- Example: 192.168.1.0/24
IPv4 requires careful address planning due to limited address space. Organizations use techniques like NAT (Network Address Translation) and VLSM to maximize efficiency.
IPv6 Subnetting
- 128-bit addresses: 340 undecillion addresses
- Common prefixes: /48, /56, /64
- Abundance: No address conservation needed
- Example: 2001:db8::/64
IPv6’s massive address space eliminates the need for NAT and enables simpler, hierarchical network designs. Most organizations receive a /48 or /56 prefix from ISPs.
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