Networking tools

IPv4 subnetting and MAC vendor lookup — all in the browser, never forwarded to an external service.

Who this collection is for

Built for system administrators, DevOps teams, IT support, developers who touch networking occasionally and students approaching IPv4 and MAC addresses without a Wireshark stack installed. It covers two tasks that come up again and again: figuring out how many addresses fit in a CIDR prefix and identifying who made a device from its MAC.

Every calculation runs entirely in your browser. There is no remote endpoint receiving the network you analyse or the MAC you pasted from a log — the code does not ship that data anywhere. It is the contract, not a promise.

From the IP range to the device

Three steps that show up when auditing a network or inspecting logs:

1.Map the address space
Start from the CIDR prefix to understand how many hosts fit, where the network and broadcast addresses live, which reservations apply (AWS VPC reserves 5 addresses, /31 falls under RFC 3021) and whether the block overlaps with an existing assignment.
2.Identify the devices that show up
When you see a MAC in a log, a DHCP lease or a tcpdump/Wireshark capture, run it through the vendor lookup: it returns the OUI if it is in the embedded snapshot, and flags locally administered, multicast or broadcast addresses before attributing a vendor.
3.Share the result with your team
Use the copy buttons: the CIDR calculator exports the line with network, broadcast and usable range; the MAC lookup copies the vendor + normalised MAC + flags summary. Both outputs are plain text, easy to paste into a ticket.

Tools in this collection

The two utilities that cover that workflow today. Both run without routing your data through a backend.

CIDR and IPv4 Subnet Calculator

CIDR / IPv4 subnet calculator with mask, wildcard, network, broadcast, usable range, FLSM splitter, VLSM planner, comparator and cheat sheet.

Turn a /n prefix into usable hosts, network and broadcast.
Check whether an IP falls inside a CIDR or whether two ranges overlap.
Plan FLSM / VLSM subnetting with copy-ready tables.

Open the CIDR calculator →

MAC vendor and OUI lookup

MAC vendor / OUI lookup with semantic detection: locally administered, multicast, broadcast and all-zero. Curated initial snapshot of the IEEE OUI registry.

Identify a vendor from a full MAC address or a 24-bit OUI.
Tell a global MAC apart from a locally administered one (privacy randomization, VMs, tunnels).
Flag broadcast and all-zero addresses instead of pretending a vendor exists.

Open the MAC lookup →

Which tool to use

Quick decisions based on what you have in front of you.
Your situation	Tool
I have a /24 prefix and want usable hosts, network and broadcast	CIDR
I need to know whether two CIDR ranges overlap	CIDR (comparator)
I have a MAC from a log or a DHCP lease	MAC lookup
I see a MAC starting with 02:, 06:, 0A: or another LAA pattern	MAC lookup (detects locally administered)
I want to geolocate a device from its MAC	Not possible with these tools — see the note below

Quick concepts

CIDR prefix: Network/length notation, e.g. 192.168.1.0/24. The number after the slash says how many leading bits identify the network. It replaces the legacy class A/B/C system.
Subnet mask: Decimal equivalent of the prefix: for /24 it is 255.255.255.0. Ones mark network bits; zeros mark host bits.
Network and broadcast: In classic IPv4 the first address of the prefix is the network address and the last is the broadcast. Neither is assignable to a host. /31 (RFC 3021) and /32 are the exceptions.
Usable hosts: Addresses you can assign to a host. For /24 it is 254 (256 − 2). AWS additionally reserves 3 more in every subnet by convention.
OUI: Organizationally Unique Identifier. The first 24 bits of a global MAC; IEEE assigns them to manufacturers. More than 36,000 prefixes in the full registry; the snapshot embedded here is a subset.
Locally administered MAC: First-octet U/L bit set to 1: the address is assigned by the operating system, not a manufacturer (Wi-Fi randomization, VMs, tunnels). Do not expect a vendor.
Multicast and broadcast: I/G bit set → multicast (a group of receivers). FF:FF:FF:FF:FF:FF is the L2 broadcast address. Common ranges: 01:00:5E:* (IPv4 multicast), 33:33:* (IPv6 multicast), 01:80:C2:* (STP), 00:00:5E:00:01:* (VRRP).

Go deeper

The cluster's editorial guides cover both questions: CIDR explains IPv4 network and subnet math (including edge cases like /31 and /32), and the MAC/OUI guide breaks down how to interpret a MAC address and why it does not geolocate devices. These tools do not geolocate devices: MAC addresses only travel on local networks.