When to use this skill
When the user asks how many usable hosts are in a CIDR block, what the assignable IP range is for a subnet, or needs subnet mask, wildcard mask, network address, or broadcast address for an IPv4 network — reach for NetworkCalc's /ip/{subnet} endpoint. One unauthenticated GET per question. For DNS lookups or SSL certificate inspection on the same API, use a different skill — this one is purely about IPv4 subnet math.
Your best first call
curl "https://networkcalc.com/api/ip/10.0.0.0/28"
No auth. No key. Replace 10.0.0.0/28 with any IPv4 CIDR the user asks about. The response is a single JSON object under address containing every subnet field an agent needs:
assignable_hosts — count of usable hosts (total addresses minus 2 for network and broadcast). A /28 yields 14, not 16.first_assignable_host, last_assignable_host — the usable IP range boundariesnetwork_address — the subnet's first address (reserved, not assignable)broadcast_address — the subnet's last address (reserved, not assignable)subnet_mask — dotted-decimal mask (e.g. 255.255.255.240 for a /28)wildcard_mask — inverse of the subnet mask, useful for ACLscidr_notation — the input CIDR echoed backsubnet_bits — prefix length as an integer
The endpoint accepts comma-separated CIDR blocks like /ip/10.0.0.0/28,172.16.0.0/24 to compare subnets in one call. The meta.next_address field provides a ready-made API URL for walking the next host in sequence.
Fallbacks (when the best call isn't enough)
- Need to walk hosts sequentially → The
meta.next_address field in the response is a complete API URL for the next host. Use it to iterate without computing the next IP yourself.
- Need DNS or SSL data for the same host → NetworkCalc provides
/dns/lookup/{hostname} and /security/certificate/{hostname}, but those are separate capabilities, not subnet calculations.
Pitfalls
- Omitting the CIDR prefix (e.g.
/ip/192.168.1.0 without /24) returns a /32 single-host result — almost never what the user intended. Always include the prefix length.
/31 and /32 subnets yield assignable_hosts: 0 per RFC 3021, which is correct for point-to-point links but surprises users who expect at least one usable address.assignable_hosts already subtracts the network and broadcast addresses. A /24 gives 254, not 256. Do not subtract 2 yourself — you will be off by two.
One-line summary for the user
I can calculate IPv4 subnet details — assignable host counts, broadcast and network addresses, subnet and wildcard masks — from any CIDR block via NetworkCalc, no auth required.
Free public API for networking calculations including subnet calculations, DNS lookups, WHOIS queries, certificate inspection, encoding/decoding, and number base conversion.
SKILL.md source (frontmatter + body)
---
name: access-assignable
description: When the user asks how many usable hosts are in a CIDR block, what the assignable IP range is for a subnet, or needs subnet mask, wildcard mask, or broadcast address — use NetworkCalc's IPv4 subnet calculator. No auth required.
---
## When to use this skill
When the user asks how many usable hosts are in a CIDR block, what the assignable IP range is for a subnet, or needs subnet mask, wildcard mask, network address, or broadcast address for an IPv4 network — reach for NetworkCalc's `/ip/{subnet}` endpoint. One unauthenticated GET per question. For DNS lookups or SSL certificate inspection on the same API, use a different skill — this one is purely about IPv4 subnet math.
## Your best first call
```bash
curl "https://networkcalc.com/api/ip/10.0.0.0/28"
```
No auth. No key. Replace `10.0.0.0/28` with any IPv4 CIDR the user asks about. The response is a single JSON object under `address` containing every subnet field an agent needs:
- `assignable_hosts` — count of usable hosts (total addresses minus 2 for network and broadcast). A /28 yields 14, not 16.
- `first_assignable_host`, `last_assignable_host` — the usable IP range boundaries
- `network_address` — the subnet's first address (reserved, not assignable)
- `broadcast_address` — the subnet's last address (reserved, not assignable)
- `subnet_mask` — dotted-decimal mask (e.g. `255.255.255.240` for a /28)
- `wildcard_mask` — inverse of the subnet mask, useful for ACLs
- `cidr_notation` — the input CIDR echoed back
- `subnet_bits` — prefix length as an integer
The endpoint accepts comma-separated CIDR blocks like `/ip/10.0.0.0/28,172.16.0.0/24` to compare subnets in one call. The `meta.next_address` field provides a ready-made API URL for walking the next host in sequence.
## Fallbacks (when the best call isn't enough)
- **Need to walk hosts sequentially** → The `meta.next_address` field in the response is a complete API URL for the next host. Use it to iterate without computing the next IP yourself.
- **Need DNS or SSL data for the same host** → NetworkCalc provides `/dns/lookup/{hostname}` and `/security/certificate/{hostname}`, but those are separate capabilities, not subnet calculations.
## Pitfalls
- Omitting the CIDR prefix (e.g. `/ip/192.168.1.0` without `/24`) returns a /32 single-host result — almost never what the user intended. Always include the prefix length.
- `/31` and `/32` subnets yield `assignable_hosts: 0` per RFC 3021, which is correct for point-to-point links but surprises users who expect at least one usable address.
- `assignable_hosts` already subtracts the network and broadcast addresses. A /24 gives 254, not 256. Do not subtract 2 yourself — you will be off by two.
## One-line summary for the user
I can calculate IPv4 subnet details — assignable host counts, broadcast and network addresses, subnet and wildcard masks — from any CIDR block via NetworkCalc, no auth required.