IPv4 Calculator

Punch in any IPv4 address and CIDR — the calculator shows the network, broadcast, host range, netmask, wildcard, address class, and the whole thing in binary.

IPv4 address

CIDR (0–32)

Overview

Binary view

network bits host bits mask = 1

Subnet Tree

Start with any block and click to split it in half (/n → two /n+1 subnets). Works down to /32. Click a node again to collapse it.

Base network

Starting CIDR

Tip: trees deeper than /28 get big fast — limit yourself before the browser does.

Tree

VLSM Planner

Give it a base block and a list of host-count requirements. It allocates the smallest subnet that fits each need, best-fit in descending size, so you don't waste address space.

Base block

Host requirements (one per line — NAME:HOSTS or just HOSTS)

Allocation

#	Name	Required	Allocated	Mask	Usable range	Broadcast	Spare

Overlap Checker

Compare two CIDRs. Tells you whether they're identical, disjoint, one contains the other, or they partially overlap — with the exact overlap range if there is one.

CIDR A

CIDR B

Result

Supernet / Summarize

Paste a list of CIDRs — one per line. Returns the shortest single CIDR that covers all of them. If no tight summary exists, it tells you honestly.

CIDRs (one per line)

Summary

IPv6 Calculator

Parse, expand, compress, and subnet any IPv6 address. Supports :: compression and full 128-bit math.

IPv6 address

Prefix (0–128)

Overview

MAC & EUI-64

Convert a 48-bit MAC address to a 64-bit interface identifier using the EUI-64 algorithm (insert FF:FE, flip the U/L bit), and see the corresponding IPv6 SLAAC address for any prefix.

MAC address

IPv6 prefix (for SLAAC)

Result

STP Root Bridge Picker

Give each switch a priority (default 32768) and MAC address. The switch with the lowest bridge ID (priority · VLAN · MAC) wins the root election. On ties, the lower MAC wins.

Switches (one per line: Name priority MAC)

How STP picks a root

Lowest priority (in steps of 4096 for Cisco PVST+/Rapid-PVST).
Tie-break on lowest MAC address.
All other switches pick one root port — the port with the lowest cost to the root.
On each segment, one designated port forwards; any remaining ports go to blocking / alternate.

EtherChannel Mode Matrix

Two switches only bundle links when their modes are compatible. PAgP and LACP cannot mix, and on only works with another on.

PAgP (Cisco)

	auto	desirable
auto	no	yes
desirable	yes	yes

PAgP negotiates. auto/auto never bundles — at least one side must be desirable.

LACP (IEEE 802.3ad)

	passive	active
passive	no	yes
active	yes	yes

LACP negotiates. passive/passive never bundles.

Static mode

Mode on does no negotiation — it just forces a bundle. It only works with another on. Mixing on with any PAgP or LACP mode will not form a channel and often causes a loop — STP has to save you.

2.4 GHz Wi-Fi Channel Planner

Each 2.4 GHz channel is 22 MHz wide but centers are only 5 MHz apart. That means most channels overlap. In North America only 1, 6, and 11 are fully non-overlapping — pick from these when deploying APs near each other.

Click any channel to highlight its 22 MHz spectrum.

Quick rules

2.4 GHz: 11 channels in US, 13 in EU. Non-overlapping trio: 1 · 6 · 11.
5 GHz: many more non-overlapping channels. 20 MHz preferred for dense deployments; 40/80/160 MHz for throughput in clean RF.
Each radio band uses separate channels — 2.4 and 5 GHz don't interfere.

Longest-Prefix Match Lookup

Routers pick the route with the most specific prefix (highest number of mask bits). Enter a routing table and a destination address — the winner is highlighted.

Routing table (prefix via next-hop, one per line)

Destination IP

Result

OSPF Cost Calculator

Cisco OSPF computes interface cost as reference-bandwidth / interface-bandwidth, rounded up, never lower than 1. The default reference is 100 Mbps — which ties every modern link at cost 1. Most shops raise it with auto-cost reference-bandwidth.

Interface bandwidth

Unit

Reference bandwidth (Mbps)

Result

Common interfaces (with default ref 100 Mbps)

Link	Bandwidth	Cost

Administrative Distance Reference

When a router learns the same destination from two protocols, it installs the route with the lowest AD. The path metric only breaks ties within one protocol — AD picks the protocol first.

Source	AD	Notes
Connected interface	0	Always wins. The router knows it directly.
Static route	1	Manually configured. Beats everything except connected.
EIGRP summary	5	Cisco EIGRP auto-summary.
External BGP (eBGP)	20	Routes from another AS.
Internal EIGRP	90	EIGRP within the same AS.
IGRP	100	Legacy Cisco, rarely seen.
OSPF	110	IETF link-state IGP.
IS-IS	115	ISP-favored link-state IGP.
RIP	120	Distance-vector, hop-count metric.
EGP	140	Obsolete exterior gateway protocol.
ODR	160	On-Demand Routing (hub-and-spoke).
External EIGRP	170	Routes redistributed into EIGRP.
Internal BGP (iBGP)	200	BGP routes learned from inside the AS.
Unknown / unreachable	255	Router will never install this.

NAT / PAT Visualizer

NAT maps between private and public addresses at the boundary between your network (inside) and the rest of the world (outside). The four address views are:

Inside localYour host's address as you see it on the LAN (usually RFC1918).

Inside globalYour host's address as the internet sees it after translation (public).

Outside globalThe remote server's real public address.

Outside localHow the remote server appears inside your network. Usually same as outside-global unless you also NAT inbound.

Translation type

Example translation table

DHCP DORA Walkthrough

Every time a device gets an IP from a DHCP server, it runs this four-message exchange. Click a step to see what is actually on the wire.

Wildcard Mask Converter

ACLs use wildcard masks instead of subnet masks. A wildcard is the bitwise inverse of the subnet mask: 0 bits must match, 1 bits are "don't care". So /24's mask 255.255.255.0 becomes wildcard 0.0.0.255.

Input (any of: CIDR 10.0.0.0/24, mask 255.255.255.0, or wildcard 0.0.0.255)

Result

ACL Builder

Build and download a Cisco IOS access-list. Standard ACLs filter by source only (1–99, 1300–1999). Extended ACLs filter by protocol + source + destination + ports (100–199, 2000–2699), and should be placed close to the source to drop traffic early.

Type

ACL name or number

Rules are applied top-down. First match wins. An implicit deny any is always at the bottom.

IOS configuration

JSON Explorer

CCNA's automation section covers JSON as the common payload for REST APIs and DNA Center / RESTCONF responses. Paste JSON and see it parsed into a collapsible tree.

JSON

Tree

Summary

Port Reference

Well-known TCP/UDP ports, filterable. Handy when reading a firewall log or writing an ACL.

Filter

Port	Proto	Service	What it does

CIDR Reference

Every IPv4 prefix from /0 to /32 with its mask, wildcard, and usable host count. The row matching your last calculator input is highlighted.

CIDR	Mask	Wildcard	/24 ratio	Addresses	Usable hosts