NetCalc Lab

IPv4/IPv6 subnetting, VLSM and route summarisation. Everything runs locally in your browser.

Subnet guide: 256 · 128 · 64 · 32 · 16 · 8 · 4 · 2 · 1
Hosts (addresses)	256	128	64	32	16	8	4	2	1
Subnet	/24	/25	/26	/27	/28	/29	/30	/31	/32
Mask (last octet)	0	128	192	224	240	248	252	254	255
Usable hosts	254	126	62	30	14	6	2	2*	1*

Each step right halves the block. Usable hosts = addresses minus 2 (network and broadcast). *A /31 is for point-to-point links (both addresses usable); a /32 is a single host.

Special address ranges (private, loopback, link-local, IPv6…)

Range	Name	From – to	What it is
10.0.0.0/8	Private	10.0.0.0 – 10.255.255.255	Home, office and lab networks. Not routed on the internet.
172.16.0.0/12	Private	172.16.0.0 – 172.31.255.255	172.16.0.0 to 172.31.255.255. Not routed on the internet.
192.168.0.0/16	Private	192.168.0.0 – 192.168.255.255	Most home routers use this. Not routed on the internet.
127.0.0.0/8	Loopback	127.0.0.0 – 127.255.255.255	Always means 'this computer' (localhost is 127.0.0.1).
169.254.0.0/16	Link-local (APIPA)	169.254.0.0 – 169.254.255.255	Self-assigned when DHCP fails. Only works on the local link.
100.64.0.0/10	Carrier-grade NAT	100.64.0.0 – 100.127.255.255	Shared address space used by ISPs.
224.0.0.0/4	Multicast	224.0.0.0 – 239.255.255.255	One sender to many listeners.
0.0.0.0/8	This network	0.0.0.0 – 0.255.255.255	0.0.0.0 means 'any / unspecified'.
192.0.2.0/24	Documentation	192.0.2.0 – 192.0.2.255	Safe example addresses (TEST-NET-1).
198.51.100.0/24	Documentation	198.51.100.0 – 198.51.100.255	Safe example addresses (TEST-NET-2).
203.0.113.0/24	Documentation	203.0.113.0 – 203.0.113.255	Safe example addresses (TEST-NET-3).
255.255.255.255/32	Limited broadcast	255.255.255.255 – 255.255.255.255	Sent to everyone on the local network.
::1/128	Loopback	::1 –	IPv6 'this computer'.
::/128	Unspecified	:: –	No address yet / any address.
fe80::/10	Link-local	fe80:: –	Automatic address on every IPv6 interface. Local link only.
fc00::/7	Unique local (ULA)	fc00:: –	IPv6 private addresses, like 10.0.0.0/8 for IPv4.
2000::/3	Global unicast	2000:: –	Normal internet-routable IPv6 addresses.
ff00::/8	Multicast	ff00:: –	One sender to many listeners (replaces broadcast).
2001:db8::/32	Documentation	2001:db8:: –	Safe example addresses for manuals and labs.
::ffff:0:0/96	IPv4-mapped	::ffff:0:0 –	An IPv4 address written inside an IPv6 address.
64:ff9b::/96	NAT64	64:ff9b:: –	Lets IPv6-only hosts reach IPv4 servers.

New to IP addressing? Start here

An IP address identifies a device on a network, like a postal address. IPv4 addresses look like 192.168.1.10; IPv6 addresses look like 2001:db8::1.

A subnet is a group of addresses that sit together on one network. The /24 after the address (CIDR notation) says how many of the address's bits are the network part. A bigger number means a smaller subnet.

Type an address with a prefix, e.g. 192.168.1.77/24. IPv4 needs the prefix; for IPv6 a missing prefix means /128 (one address).

Address / prefix (IPv4 or IPv6)

Network: 192.168.1.0/24
Netmask: 255.255.255.0
Wildcard: 0.0.0.255
Broadcast: 192.168.1.255
First usable: 192.168.1.1
Last usable: 192.168.1.254
Total addresses: 256
Usable hosts: 254

Binary view: dark boxes are network bits (fixed), light boxes are host bits (free to change). Click a box, or use the slider, to move the boundary.

Prefix length: /24 (24 network bits, 8 host bits)

How was this calculated? (step by step)

An IPv4 address is really one 32-bit number, written as four 8-bit chunks (octets) for humans. 192.168.1.77 in binary is:
```
11000000.10101000.00000001.01001101
```
The "/24" means the first 24 bits identify the network (which street you are on) and the remaining 8 bits identify the host (which house on that street). The mask is 24 ones followed by 8 zeros:
```
11111111.11111111.11111111.00000000
= 255.255.255.0
```
Network address: keep the network bits and set every host bit to 0 (a bitwise AND of the address and the mask). The bar "|" below marks the split.
```
address 11000000.10101000.00000001|01001101
network 11000000.10101000.00000001|00000000
= 192.168.1.0
```
Broadcast address: keep the network bits and set every host bit to 1. Packets sent here reach everyone on the subnet.
```
broadcast 11000000.10101000.00000001|11111111
= 192.168.1.255
```
Wildcard mask is the mask flipped (ones become zeros). Routers and access lists use it to say "ignore these bits": 0.0.0.255.
Total addresses = 2 to the power of the host bits = 2^8 = 256.
Usable hosts = total - 2, because the first address (192.168.1.0) names the network itself and the last (192.168.1.255) is the broadcast. So 256 - 2 = 254, from 192.168.1.1 to 192.168.1.254.

New to IP addressing? Start here

Splitting cuts one large network into several equal smaller ones, for example one office block into separate VLANs for staff, guests and printers.

Enter the big network and the longer prefix you want. The table shows each resulting subnet.

Parent network

New prefix length

4 subnet(s)

Subnet	First	Last
192.168.0.0/26	192.168.0.0	192.168.0.63
192.168.0.64/26	192.168.0.64	192.168.0.127
192.168.0.128/26	192.168.0.128	192.168.0.191
192.168.0.192/26	192.168.0.192	192.168.0.255

How was this calculated? (step by step)

Splitting means moving the network/host boundary to the right. You go from /24 to /26, so you "borrow" 26 - 24 = 2 bits from the host part and use them to number the new subnets.
Each borrowed bit doubles the number of subnets: 2^2 = 4 subnets.
Each new subnet has 6 host bits, so it holds 2^6 = 64 addresses (62 usable). This is also the "step": each subnet starts that many addresses after the previous one.
Subnets never overlap and always start on a multiple of their size. In the last octet that is a step of 64: 0, 64, 128... The table lists each subnet with its first and last address.

Saved lab plans (this browser only)

Plan name

No saved plans here yet.

Import plans from JSON

New to IP addressing? Start here

Real networks are uneven: a user LAN may need 100 addresses while a router link needs 2. VLSM gives each its own right-sized subnet from one parent block.

Enter the parent block you own, then each subnet's name and how many hosts (devices) it must hold. The app orders and places them for you.

Saved plans are stored only in this browser on this computer.

Parent block (IPv4)

Requirements

Largest subnets are placed first. If two need the same size, the one higher in this list goes first. Growth adds headroom, for example 20% turns 100 hosts into 120.

Name

Hosts

Growth %

Name

Hosts

Growth %

Name

Hosts

Growth %

VLSM plan for 192.168.10.0/24

Parent block use: 63% allocated, 94 addresses free from 192.168.10.162.

Name	Hosts	Subnet	Mask	Usable range	Broadcast
LAN A	100/126	192.168.10.0/25	255.255.255.128	192.168.10.1 – 192.168.10.126	192.168.10.127
LAN B	30/30	192.168.10.128/27	255.255.255.224	192.168.10.129 – 192.168.10.158	192.168.10.159
WAN link	2/2	192.168.10.160/31	255.255.255.254	192.168.10.160 – 192.168.10.161	192.168.10.161

How was this calculated? (step by step)

VLSM (Variable Length Subnet Masking) lets each subnet be exactly as big as it needs to be instead of forcing them all to the same size. That wastes far fewer addresses.
Step 1 - size each subnet. A subnet with N hosts needs N + 2 addresses (network + broadcast), rounded up to the next power of two. The number of host bits is the exponent, and the prefix is 32 minus the host bits. Special cases: 1 host fits a /32 and 2 hosts fit a /31.
Step 2 - place the biggest first. Sorting largest to smallest keeps every subnet aligned to its own size boundary and leaves no gaps. We start at the beginning of 192.168.10.0/24 (/24) and move forward.
LAN A: 100 hosts + 2 = 102. Smallest power of two that fits is 2^7 = 128, so the prefix is /25 (mask 255.255.255.128). Placed at 192.168.10.0/25; usable 192.168.10.1 - 192.168.10.126, broadcast 192.168.10.127. That gives 126 usable addresses for 100 requested.
LAN B: 30 hosts + 2 = 32. Smallest power of two that fits is 2^5 = 32, so the prefix is /27 (mask 255.255.255.224). Placed at 192.168.10.128/27; usable 192.168.10.129 - 192.168.10.158, broadcast 192.168.10.159. That gives 30 usable addresses for 30 requested.
WAN link: 2 hosts is a special case. Smallest power of two that fits is 2^1 = 2, so the prefix is /31 (mask 255.255.255.254). Placed at 192.168.10.160/31; usable 192.168.10.160 - 192.168.10.161. That gives 2 usable addresses for 2 requested.
Whatever is left over, from 192.168.10.162 onward, is free for future subnets.

Saved lab plans (this browser only)

Plan name

No saved plans here yet.

Import plans from JSON

New to IP addressing? Start here

Summarising turns many routes into one, so routers keep shorter tables. It works best when your subnets were allocated next to each other and aligned.

Enter routes one per line (CIDR format). All must be IPv4 or all IPv6.

Routes, one per line (all IPv4 or all IPv6)

Summary route: 192.168.0.0/22
Exact?: Yes — covers only the listed routes
Minimal exact set: 192.168.0.0/22

How was this calculated? (step by step)

Route summarisation (aggregation) replaces many routes with one shorter route that covers them all. Routers then keep a smaller routing table and advertise less.
Method: write the lowest and the highest address in binary and find how many leading bits they share. Those shared bits are the summary's network prefix.
Lowest network and highest address across your routes (the bar marks the shared prefix of /22):
```
lowest  11000000.10101000.000000|00.00000000
highest 11000000.10101000.000000|11.11111111
```
They share the first 22 bits, so the summary route is 192.168.0.0/22.
Every address inside 192.168.0.0/22 belongs to one of your routes, so this summary is exact: no traffic will be attracted that you cannot deliver.
Minimal exact set: neighbouring routes of equal size that form one bigger aligned block are merged repeatedly. This covers precisely your routes with the fewest entries: 192.168.0.0/22.

Saved lab plans (this browser only)

Plan name

No saved plans here yet.

Import plans from JSON

New to IP addressing? Start here

IPv6 addresses are long, so they have a short form: drop leading zeros in each group and replace one run of all-zero groups with ::. Paste either form to see both.

The site planner splits an IPv6 block (usually a /48) into /64 networks per site. Every LAN should be a /64.

Expand and compress

IPv6 address (optionally with /prefix)

Compressed: 2001:db8::1
Expanded: 2001:0db8:0000:0000:0000:0000:0000:0001

Site planner: split a block into /64 networks

Parent block (/64 or shorter, e.g. a /48)

Each site gets an aligned block of /64s, rounded up to a power of two (5 LANs gets room for 8).

Site

/64 networks

Site

/64 networks

Site

/64 networks

Site	Needs	Block	Capacity	First /64	Last /64
Lab	16	2001:db8:1::/60	16	2001:db8:1::/64	2001:db8:1:f::/64
HQ	5	2001:db8:1:10::/61	8	2001:db8:1:10::/64	2001:db8:1:17::/64
Branch	1	2001:db8:1:18::/64	1	2001:db8:1:18::/64	2001:db8:1:18::/64

65,511 /64 networks still free.