What is a subnet calculator and why do I need one?

A subnet calculator is a tool that helps network engineers and developers calculate IP address ranges, CIDR notation, subnet masks, and network boundaries. It's essential for proper network planning, security segmentation, and avoiding IP conflicts in cloud environments like AWS, Azure, and GCP.

How do I calculate CIDR notation manually?

CIDR notation uses a slash followed by a number (like /24) to indicate how many bits are used for the network portion. For example, /24 means the first 24 bits are for the network, leaving 8 bits for hosts (256 total IPs, 254 usable). Our calculator does this instantly with visual representations.

What's the difference between network address and broadcast address?

The network address is the first IP in a subnet (like 192.168.1.0) and identifies the network itself - you cannot assign it to devices. The broadcast address is the last IP (like 192.168.1.255) used for broadcasting messages to all devices in the subnet - also cannot be assigned to devices.

How many IP addresses can I use in a /24 subnet?

A /24 subnet contains 256 total IP addresses, but only 254 are usable for devices. You lose 2 addresses: the network address (first) and broadcast address (last). In cloud environments like AWS, you lose 5 addresses due to additional reservations.

Does this subnet calculator work for AWS VPC planning?

Yes! Our calculator specifically accounts for AWS VPC reserved IP addresses. AWS reserves the first 4 IPs and the last IP in every subnet, so a /24 subnet in AWS only provides 251 usable IPs instead of 254. We show both standard and AWS-specific calculations.

Can I use this calculator for IPv6 subnetting?

Currently, our calculator focuses on IPv4 subnetting, which covers 99% of network planning needs. IPv6 uses a different addressing scheme with much larger address spaces, and we're considering adding IPv6 support in future updates.

Is my network information secure when using this calculator?

Absolutely! All calculations happen entirely in your browser using JavaScript. No network information is sent to our servers or stored anywhere. Your IP addresses and network details remain completely private and secure.

What's the largest subnet I can calculate?

You can calculate subnets from /1 (over 2 billion IPs) down to /32 (single host). However, practical subnets typically range from /8 (16 million IPs) for large enterprises down to /30 (2 IPs) for point-to-point links.

Free Routing Table Simulator - Learn Network Routing Decisions

Routing Table Simulator

Network Scenario

Current Routing Table

Network	Next Hop	Interface	Protocol	Metric	Admin Distance
203.0.113.0/30	directly connected	wan0	connected	0	0
192.168.1.0/24	directly connected	eth0	connected	0	0
10.0.0.0/24	directly connected	eth1	connected	0	0
172.16.0.0/16	10.0.0.2	eth1	static	1	1
0.0.0.0/0	203.0.113.1	wan0	static	1	1

Network Interfaces

Interface	IP Address	Network	Status
eth0	192.168.1.1/24	192.168.1.0/24	up
eth1	10.0.0.1/24	10.0.0.0/24	up
wan0	203.0.113.2/30	203.0.113.0/30	up

Packet Forwarding Simulation

Add Custom Route

How Routing Works

Longest Prefix Match

Routers use the most specific (longest prefix) route that matches the destination IP address.

Example: For destination 192.168.1.100, a /24 route is preferred over a /16 route.

Administrative Distance

When multiple routes have the same prefix length, the route with the lowest administrative distance is preferred.

Connected (0) > Static (1) > EIGRP (90) > OSPF (110) > RIP (120)

Metric

If routes have the same prefix length and administrative distance, the route with the lowest metric is chosen.

Metrics represent the "cost" of using a particular path.

Default Route

A route to 0.0.0.0/0 matches all destinations and is used when no more specific route exists.

Often called the "gateway of last resort".

Understanding Router Forwarding Decisions

Routing Table Basics

Network: Destination network address and prefix length
Next Hop: IP address of the next router in the path
Interface: Local interface to use for forwarding
Metric: Cost or distance to the destination
Protocol: How the route was learned

Route Selection Process

1. Longest Prefix Match: Most specific route wins
2. Administrative Distance: Most trusted source
3. Metric: Lowest cost path
4. Load Balancing: Equal cost paths (if supported)

Common Routing Scenarios

🏠 Basic Home/Office

Simple network with:

• Local LAN networks (192.168.x.x, 10.x.x.x)
• Single internet connection
• Default route to ISP
• Static routes for remote offices

🏢 Enterprise Network

Complex network with:

• Multiple LAN segments
• OSPF for internal routing
• Redundant WAN connections
• Load balancing and failover

🌐 ISP/Service Provider

Internet-scale routing with:

• BGP for internet routing
• Customer route aggregation
• Upstream provider connections
• Traffic engineering

🔄 Redundant Paths

High availability with:

• Primary and backup routes
• Different metrics for failover
• Automatic path selection
• Network resilience

Routing Protocol Comparison

Protocol	Type	Admin Distance	Metric	Use Case
Connected	Direct	0	0	Directly attached networks
Static	Manual	1	Manual	Small networks, backup routes
EIGRP	Distance Vector	90	Composite	Cisco networks, fast convergence
OSPF	Link State	110	Cost (bandwidth)	Enterprise networks, scalable
RIP	Distance Vector	120	Hop Count	Simple networks, legacy
BGP	Path Vector	20 (eBGP)	Path Attributes	Internet routing, ISPs

Learning Tips

Start Simple: Begin with the basic scenario to understand fundamentals
Test Different IPs: Try various destination addresses to see how routing changes
Compare Scenarios: Switch between scenarios to see different routing approaches
Add Custom Routes: Experiment with your own routes to see their effects
Follow the Steps: Read the decision process to understand router logic
Practice Subnetting: Use our subnet calculator to understand network boundaries

Routing Table Simulator

Routing Table Simulator

Network Scenario

Current Routing Table

Network Interfaces

Packet Forwarding Simulation

How Routing Works

Longest Prefix Match

Administrative Distance

Metric

Default Route

Understanding Router Forwarding Decisions

Routing Table Basics

Route Selection Process

Common Routing Scenarios

🏠 Basic Home/Office

🏢 Enterprise Network

🌐 ISP/Service Provider

🔄 Redundant Paths

Routing Protocol Comparison

Learning Tips

Related Network Tools

Subnet Calculator

VLSM Calculator

Bandwidth Calculator

IP Address Validator