This page is a containment validator: is this IPv6 address range fully inside that CIDR block? It’s a fast, low-risk check before you commit changes to firewalls, routing, or IPAM. If you searched for “IPv6 CIDR to Range Calculator” but what you really need is “does this range fit,” this tool is the direct answer.

Key Features

Full containment validation: confirms both endpoints (start and end) are inside the CIDR network.
Boundary detection: catches ranges that straddle the next subnet boundary (a common off-by-one mistake).
CIDR notation support: works with standard address/prefix formats.
Operationally useful output: designed for day-to-day tasks like allowlists, ACL objects, and inventory cleanup.

Use Cases

Firewall / WAF allowlists: ensure the permitted range is inside the intended subnet and doesn’t widen access unintentionally.
Cloud network allocation: validate that a requested child block fits within the parent VPC/VNet range you reserved.
IPAM documentation checks: verify that recorded ranges actually match the CIDR they’re filed under.
Single IP membership: if you only need to validate one address (not an entire range), use address-in-range instead.

How To Use

Enter the range start (example: 2001:db8::10).
Enter the range end (example: 2001:db8::ff).
Enter the CIDR network (example: 2001:db8::/48).
Run the check:

In range means the entire range is contained.
Not in range means at least one endpoint escapes the CIDR block, or the range is invalid (start > end).

Result interpretation tip: if your start is inside but your end is outside, you probably meant a different prefix length (for example /23 instead of /24) or you captured the first address of the next subnet.

How It Works

A CIDR block represents a contiguous set of addresses. Containment is true when:

both endpoints are members of the network, and
the start address is less than or equal to the end address.

In practice, the tool normalizes the CIDR into a network boundary and compares both endpoints against that boundary. If you’re still deciding what the parent CIDR should be (rather than validating it), start with a calculator view like subnet calculator to see network/broadcast/host ranges and block sizes.

Examples

CIDR range checker: 2001:db8::10–2001:db8::ff inside 2001:db8::/48

Expected: in range (both endpoints sit within the parent block).

CIDR range table sanity check: does 10.0.1.0–10.0.1.255 fit inside 10.0.0.0/23?

Yes—/23 covers two consecutive /24 blocks.

IP range calculator boundary failure: range 192.168.1.250–192.168.2.10 inside 192.168.1.0/24

Expected: not in range because the end address is in the next /24.

Excel-style bulk validation: if you’re validating hundreds of entries, spot-check a few examples online first, then automate with a script and compare.

python

import ipaddress
net = ipaddress.ip_network("2001:db8::/48", strict=False)
start = ipaddress.ip_address("2001:db8::10")
end = ipaddress.ip_address("2001:db8::ff")
contained = (start in net) and (end in net) and (int(start) <= int(end))
print("contained:", contained)

Edge Cases & Troubleshooting

Start/end swapped: IPv6 ranges must be ordered. If start > end, the range is invalid.
Boundary straddle: the most common failure—your range overlaps two adjacent subnets because the end address is just past the boundary.
Prefix confusion: /16, /24, /25 mistakes are frequent when copying rules between environments.
Formatting and typos: extra spaces, missing octets/hextets, or non-decimal characters will cause parsing errors.

Reverse Intent

If you have the opposite requirement—convert a range into the smallest set of CIDR blocks that covers it—that’s a conversion problem, not containment. After converting, run containment checks again to confirm you didn’t accidentally widen the allowed space.

FAQ

What’s the difference between a CIDR block and an IP range?

CIDR is defined by a prefix (network + /n). A range is defined by a start and end. A CIDR block always maps to one contiguous range; a manually-entered range may not align to subnet boundaries.

What is IPv6 address in CIDR format?

It’s an address written with a prefix length (example: 192.168.1.10/24 or 2001:db8::1/64).

What is 255.255.255.0 in CIDR format?

It is /24.

What is the range of 100.64.0.0/10?

It runs from 100.64.0.0 through 100.127.255.255 (CGNAT space). Use this tool to confirm whether a smaller range is contained within it.

Next Steps / Related Workflows

If the range is not contained, either adjust the parent CIDR or split your range so each segment aligns to a boundary.
Keep an audit trail of your allocations (CIDR, purpose, owner, environment) so allowlists and route summaries stay maintainable.
When you need the complementary check in the other address family or for adjacent workflows, continue with the related tool.

Make everything as simple as possible, but not simpler.
Albert Einstein