Nth IP Calculator

The Nth IP Calculator finds the IP address at a specific index position within a CIDR network or IP range. This tool is invaluable for subnet planning, IP allocation scripting, and quickly identifying specific addresses without enumerating entire ranges. It supports flexible syntax with multiple delimiters (@, [], or space) and works with both IPv4 and IPv6 networks. Whether you need the 10th address in 192.168.1.0/24 or the 1000th address in a /64 IPv6 subnet, this calculator provides instant results with bounds checking and clear error messages.

The Nth IP Calculator performs indexed address lookups within IP networks, treating each network as a zero-indexed array of IP addresses.

Indexing Convention:

• Index 0: First IP address in the range/CIDR
• Index 1: Second IP address (often the gateway in IPv4)
• Index -1: Last IP address (may support negative indices)
• Maximum index = (total IPs in range) - 1

Syntax Formats:

• @ delimiter: 192.168.1.0/24 @ 10
• [] delimiter: 192.168.1.0/24 [10]
• Space delimiter: 192.168.1.0/24 10
• All formats are equivalent and produce the same result

IPv4 Examples:

• 192.168.1.0/24 @ 0 → 192.168.1.0 (network address)
• 192.168.1.0/24 @ 1 → 192.168.1.1 (often gateway)
• 192.168.1.0/24 @ 10 → 192.168.1.10
• 192.168.1.0/24 @ 255 → 192.168.1.255 (broadcast address)
• 10.0.0.1-10.0.0.10 @ 0 → 10.0.0.1 (first in range)
• 10.0.0.1-10.0.0.10 @ 9 → 10.0.0.10 (last in range)

IPv6 Examples:

• 2001:db8::/64 @ 0 → 2001:db8:: (subnet address)
• 2001:db8::/64 @ 1 → 2001:db8::1 (often router)
• 2001:db8::/64 @ 1000 → 2001:db8::3e8
• fe80::/10 @ 100 → fe80::64

Global Offset:

The global offset is added to all specified indices, useful for systematic calculations:

• Query: 192.168.1.0/24 @ 10, Global offset: 5 → Index 15 → 192.168.1.15
• Query: 10.0.0.0/24 @ 0, Global offset: 1 → Index 1 → 10.0.0.1
• Useful for skipping network addresses or finding blocks of addresses

Bounds Checking:

• Tool validates that index is within valid range
• Error shown if index exceeds (total IPs - 1)
• Maximum index displayed for reference
• Example error: "Index 300 out of bounds (max: 255)" for /24 subnet

Common IP Allocation Patterns:

• Index 0: Network address (IPv4) or subnet prefix (IPv6)
• Index 1: Gateway/router (common convention)
• Indices 2-10: Static servers (DNS, NTP, etc.)
• Indices 11+: DHCP pool or dynamic hosts
• Last index: Broadcast (IPv4) or highest address (IPv6)

Performance Considerations:

The tool efficiently calculates addresses without enumerating all IPs in the range. Even for massive IPv6 /64 subnets (2^64 addresses), index-based lookup is instant through mathematical calculation rather than iteration.

Calculation Method:

1. Parse network/range and convert to start IP + size
2. Add index to start IP address (big integer arithmetic)
3. Validate result is within network bounds
4. Convert back to IP address string format

Use in Automation Scripts:

This calculator pattern is commonly used in infrastructure-as-code:

• Ansible: Assigning IPs based on inventory position
• Terraform: Calculating static IPs for resources
• Python ipcalc: subnet[10] notation
• Go net package: IP arithmetic for allocation

IPv4 vs. IPv6 Differences:

• IPv4: Index 0 = network, last index = broadcast (for classful subnets)
• IPv6: No broadcast concept, all addresses potentially usable
• IPv6: Subnets are typically /64, providing 2^64 addresses (practically unlimited)
• IPv4: Small subnets (/30, /31) have special considerations