Subnet Calculator (IPv4 + IPv6)

Calculate subnet details for both IPv4 and IPv6 networks. Supports CIDR notation, provides usable host ranges, neighboring subnets, address classification, and standard allocation annotations.

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 "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"]}'

subnet-calculator