subnet-calculator

name	subnet-calculator
description	IPv4 and IPv6 subnet calculator - CIDR breakdown, usable hosts, previous/next subnets, address classification, VLSM planning, and dual-stack analysis. Use when calculating subnets, figuring out how many hosts fit in a prefix, planning IP addressing, getting wildcard masks for ACLs, or checking if two IPs are in the same subnet.
license	Apache-2.0
user-invocable	true
metadata	{"openclaw":{"requires":{"bins":["python3"],"env":["SUBNET_MCP_SCRIPT","MCP_CALL"]}}}

Subnet Calculator (IPv4 + IPv6)

Available Tools

1. subnet_calculator — IPv4 Subnet Details

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"192.168.1.0/24"}'

Parameters:

• cidr (required): IPv4 CIDR notation, e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.1.0/24

Returns:

• Network address, broadcast address, netmask, wildcard mask
• Prefix length, host bits
• Number of total and usable addresses
• First and last usable host addresses
• Usable hosts preview (up to 10)
• Previous and next subnets (same size)
• Address classification: private, global, link-local, multicast, loopback, reserved
• Human-readable summary

2. subnet_calculator_v6 — IPv6 Subnet Details

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8::/48"}'

Parameters:

• cidr (required): IPv6 CIDR notation, e.g., 2001:db8::/32, fd00::/64, fe80::/10

Returns:

• Network address (compressed and exploded forms)
• Last address in range
• Prefix length, host bits
• Number of addresses (exact for /64+, exponential notation for larger)
• Number of /64 subnets contained
• Previous and next subnets
• Address classification: ULA, global unicast, link-local, multicast
• Standard allocation annotation (e.g., "/64 = SLAAC capable", "/48 = site allocation")
• Human-readable summary

3. subnet_calculator_auto — Auto-Detect IPv4/IPv6

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_auto '{"cidr":"10.0.0.0/24"}'
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_auto '{"cidr":"2001:db8:abcd::/48"}'

Automatically detects IP version and calls the appropriate calculator.

When to Use

• Interface addressing: Calculate the correct subnet for a new interface
• VLSM planning: Break a large block into appropriately sized subnets
• ACL wildcard masks: Get the wildcard mask for access-list entries
• Routing verification: Confirm that route entries match expected subnets
• IPv6 migration planning: Understand IPv6 allocation standards (/48, /64, /128)
• Network design: Validate addressing schemes before deployment
• Troubleshooting: Verify if two IPs are in the same subnet

Common Network Engineering Scenarios

Scenario 1: Point-to-Point Link Addressing

IPv4 /30 link:

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.1.1.0/30"}'

Result: 4 addresses, 2 usable (10.1.1.1 and 10.1.1.2). Standard for router-to-router links.

IPv4 /31 link (RFC 3021):

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.1.1.0/31"}'

Result: 2 addresses (10.1.1.0 and 10.1.1.1). No broadcast waste.

IPv6 /127 link (RFC 6164):

python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:1::/127"}'

Result: 2 addresses. Recommended for IPv6 point-to-point links.

Scenario 2: VLSM Subnet Planning

Break 10.10.0.0/16 into subnets for different departments:

# Engineering: 500 hosts needed → /23 (510 usable)
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.10.0.0/23"}'

# Sales: 100 hosts needed → /25 (126 usable)
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.10.2.0/25"}'

# Management: 10 hosts needed → /28 (14 usable)
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.10.2.128/28"}'

# Server VLAN: 30 hosts needed → /27 (30 usable)
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.10.2.192/27"}'

Produce an addressing plan:

Subnet Plan — 10.10.0.0/16
┌────────────┬─────────────────┬────────┬───────────┬───────────────┐
│ Department │ Subnet          │ Prefix │ Usable    │ Gateway       │
├────────────┼─────────────────┼────────┼───────────┼───────────────┤
│ Engineering│ 10.10.0.0/23    │ /23    │ 510 hosts │ 10.10.0.1     │
│ Sales      │ 10.10.2.0/25    │ /25    │ 126 hosts │ 10.10.2.1     │
│ Management │ 10.10.2.128/28  │ /28    │ 14 hosts  │ 10.10.2.129   │
│ Servers    │ 10.10.2.192/27  │ /27    │ 30 hosts  │ 10.10.2.193   │
│ P2P Links  │ 10.10.3.0/24    │ /30 ea │ 2 per link│ varies        │
└────────────┴─────────────────┴────────┴───────────┴───────────────┘

Scenario 3: IPv6 Site Allocation

Plan a /48 allocation for a campus:

# Site allocation
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:abcd::/48"}'

# Building 1 — first /56 from the /48
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:abcd::/56"}'

# Floor 1, Building 1 — first /64 from the /56
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:abcd::/64"}'

# Loopback /128
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:abcd::1/128"}'

# Point-to-point /127
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:abcd:ffff::/127"}'

IPv6 Allocation — 2001:db8:abcd::/48
┌──────────────┬──────────────────────────┬────────┬──────────────────────┐
│ Purpose      │ Prefix                   │ Size   │ Note                 │
├──────────────┼──────────────────────────┼────────┼──────────────────────┤
│ Site         │ 2001:db8:abcd::/48       │ /48    │ 65,536 /64 subnets   │
│ Building 1   │ 2001:db8:abcd::/56       │ /56    │ 256 /64 subnets      │
│ Floor 1/B1   │ 2001:db8:abcd::/64       │ /64    │ SLAAC capable        │
│ Loopback     │ 2001:db8:abcd::1/128     │ /128   │ Single host          │
│ P2P Link     │ 2001:db8:abcd:ffff::/127 │ /127   │ RFC 6164             │
└──────────────┴──────────────────────────┴────────┴──────────────────────┘

Scenario 4: ACL Wildcard Mask Reference

# What's the wildcard for a /22?
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.0.0.0/22"}'

Use the wildcard_mask field directly in ACL configuration:

ip access-list extended EXAMPLE
  permit ip 10.0.0.0 0.0.3.255 any

Scenario 5: Dual-Stack Verification

Verify both IPv4 and IPv6 assignments on an interface:

# IPv4 side
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.1.1.1/30"}'

# IPv6 side
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator_v6 '{"cidr":"2001:db8:1::1/127"}'

Quick Reference: Common Prefix Sizes

IPv4

Prefix	Hosts	Usable	Use Case
/30	4	2	Point-to-point link
/29	8	6	Small DMZ
/28	16	14	Management VLAN
/27	32	30	Server VLAN
/26	64	62	Small department
/25	128	126	Medium department
/24	256	254	Standard subnet
/23	512	510	Large subnet
/22	1024	1022	Campus building
/16	65536	65534	Campus site

IPv6

Prefix	Subnets (/64)	Use Case
/128	0	Single host (loopback)
/127	0	Point-to-point link (RFC 6164)
/64	1	Standard subnet (SLAAC)
/56	256	Building or floor
/48	65,536	Site allocation
/32	16,777,216	ISP allocation

Integration with Network Config

After calculating subnets, use pyats-config-mgmt to apply:

# Calculate the subnet first
python3 $MCP_CALL "python3 -u $SUBNET_MCP_SCRIPT" subnet_calculator '{"cidr":"10.1.1.0/30"}'

# Then configure the interface
PYATS_TESTBED_PATH=$PYATS_TESTBED_PATH python3 $MCP_CALL "${PYATS_PYTHON:-python3} -u $PYATS_MCP_SCRIPT" pyats_configure_device '{"device_name":"R1","config_commands":["interface GigabitEthernet2","ip address 10.1.1.1 255.255.255.252","no shutdown"]}'