MTU and IP Fragmentation

Type packet, MTU, and header. 2000, 1500 and 20 give 2. 1400, 1500 and 20 give 1. 9000, 1500 and 40 give 7. Header math, not path MTU.

Payload = MTU minus header, then ceil(packet / payload). Not a PMTUD probe. IPv4 hosts sit on the IP Subnet / CIDR Calculator.

Input data

Quick MTU:
Options

Default IPv4 header = 20 B. Presets only fill MTU — results after Calculate.

Results

Enter data and click Calculate.

How it works

MTU and IP Fragmentation in this calculator counts fragments from three numbers. 2000 B, MTU 1500 and header 20 give 2. 1400, 1500 and 20 give 1. 9000, 1500 and header 40 give 7. Payload = MTU minus header. This is not path MTU.

Fields: mtu-pkt, mtu-mtu, mtu-ip-header. 1500 minus 20 = 1480. ceil(2000 / 1480) = 2. 1400 fits in 1480, so 1. 1500 minus 40 = 1460. ceil(9000 / 1460) = 7. Presets 1500, 1492, 1400 only fill MTU.

2 is not a ping -M do result. 1 does not come from traceroute. 7 is not a PMTUD probe. You type the header: 20 for IPv4, 40 for IPv6. The calculator does not query the path.

IP Subnet / CIDR next door counts hosts. IPv6 shorthand compresses hextets. Here 2000 and 1500 stay 2, header math alone.

Type 2000, 1500 and 20, then Calculate. The result is 2. A comma is not needed. Zero MTU does not divide. This is not live fragmentation.

2000, 1500 and 20 give 2. 1400, 1500 and 20 give 1. 9000, 1500 and 40 give 7. Another header at 2000 changes 2.

Formula

payload = MTU − header; fragments = ceil(packet / payload), or 1 when the packet fits. Header math, not PMTU.

How to use

  1. Type packet 2000, MTU 1500, and header 20.
  2. Click Calculate. Fragments are 2.
  3. 1400 at 1500 and 20 gives 1. 9000 at 1500 and 40 gives 7.
  4. Header math, not path MTU.
  5. IPv4 hosts sit on the IP Subnet / CIDR Calculator.

2000 B, MTU 1500 and 20 = 2

Payload = MTU minus header. 2000, 1500 and 20 give 2. Header math, not PMTU.

MTU
Field mtu-mtu. 1500 at 2000 and 20 leaves 2. Not a path probe.
Fragmentation
ceil(packet / payload). 9000 and header 40 give 7.
header
Field mtu-ip-header. 20 or 40. 1400 at 20 stays 1.

Examples

Example 1

  • packet 2000 B
  • MTU 1500
  • header 20

2

How many fragments at 2000 B, MTU 1500 and header 20? 2. Header math, not PMTU.

Example 2

  • packet 1400 B
  • MTU 1500
  • header 20

1

What about 1400 B, MTU 1500 and 20? 1. The packet fits.

Example 3

  • packet 9000 B
  • MTU 1500
  • header 40

7

What about 9000 B, MTU 1500 and header 40? 7.

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Common questions

How many fragments at 2000 B, 1500 and 20?

2. Payload 1480, ceil(2000 / 1480). Not PMTU.

What about 1400 B, 1500 and 20?

1. 1400 fits in 1480.

What about 9000 B, 1500 and 40?

7. Payload 1460, ceil(9000 / 1460).

Is this a path MTU probe?

No. Three typed numbers. No ICMP and no PMTUD.

Which header do I type?

20 for plain IPv4. 40 for base IPv6. Tunnels often lower the MTU field.

Do presets calculate on their own?

No. 1500, 1492 and 1400 only fill MTU. Then Calculate.

Is 2 a DF or black-hole flag?

No. The calculator does not know DF. Fragment count only.

Where do I count CIDR hosts?

On the IP Subnet / CIDR Calculator. Here 2 from 2000 and 1500 stays.

Does zero MTU count?

No. MTU and packet must be positive. This is not an empty frame from a test.

Knowledge sources

The calculator counts bits, bytes or throughput from your numbers. Below are SI and bit definitions (NIST).

Page updated in 2026.

What MTU means

MTU (Maximum Transmission Unit) is the largest packet/frame size (in bytes) a link or path can carry without splitting. On typical Ethernet, IPv4 MTU is usually 1500 B — the limit for an IP datagram (IP header + payload) that fits an Ethernet frame without fragmentation on that hop.

If you send a packet larger than the path MTU, a router must fragment it (when allowed) or drop it (when the DF — Don't Fragment — bit is set). This calculator estimates how many fragments result for a given packet size and MTU, and the usable per-fragment payload (MTU − IP header).

What IP fragmentation is

  • A large datagram is split into smaller fragments, each with its own IP header.
  • The receiver reassembles them; losing one fragment loses the whole datagram.
  • Fragmentation adds header overhead, CPU cost, and fragility (middleboxes that mishandle fragments).
  • In IPv6, intermediate routers do not fragment — the source does; path MTU discovery matters even more.

Common MTU values

  • 1500 — classic Ethernet / most LAN and many WAN links.
  • 1492 — common for PPPoE (Ethernet 1500 minus ~8 B PPPoE overhead).
  • ~1400–1420 — typical VPN tunnels (IPsec, OpenVPN, WireGuard depending on overhead); exact value varies.
  • 9000 — jumbo frames in some datacenter LANs (must be consistent end-to-end).
  • 1280 — minimum required for IPv6.

Form presets (1500 / 1492 / 1400) cover the most common LAN, PPPoE, and “safe” VPN starting points.

Example calculations

Assume IPv4 header = 20 B (no options). Per-fragment payload ≈ MTU − 20.

  • 2000 B packet, MTU 1500: first fragment carries up to ~1480 B payload + 20 B IP; the rest follows. Expect 2 fragments (calculator simplification — real splits also honor fragment offset alignment).
  • 1500 B packet, MTU 1500: fits in one piece — 1 (no fragmentation at that limit).
  • 1500 B packet, MTU 1492 (PPPoE): no longer fits — fragmentation or lower MSS/MTU at the source.
  • 1400 B packet, MTU 1400 (VPN): tunnel edge case; larger TCP flows often need MSS clamping.

Enter your own numbers — the result shows fragment count and per-frame payload.

Quick table: MTU vs packet size

A simplified view — use the form for exact fragment counts (IP header + offset alignment).

MTUPacketFits?Note
15001400Yes (1)Typical Ethernet — headroom
15001500Yes (1)Ethernet MTU boundary
15001600No → fragmentsExpect ≥2 fragments
14921500No → fragmentsCommon PPPoE case
14001500No → fragmentsCommon VPN starting point

Educational arithmetic, not full Path MTU Discovery. With DF and black-hole symptoms, diagnose separately (ping/tracepath/VPN MSS).

Why fragmentation matters

  • Performance — more packets mean more interrupts and headers.
  • Reliability — one lost fragment kills the datagram; TCP retransmits more.
  • Security / middleboxes — some firewalls mishandle or drop fragments.
  • Symptoms — partial page loads, VPN stalls on large transfers, SSH works but HTTPS does not (classic MTU black hole).

Honest note: this is simplified, not full PMTUD

The calculator does arithmetic splitting by MTU and IP header size. It does not simulate Path MTU Discovery, DF bits, ICMP “Packet Too Big”, TCP MSS clamping, or real-world IPv4/IPv6 operator quirks. Treat the output as a quick educational/planning estimate — for production diagnostics use ping -f -l / tracepath / VPN tests per your link docs.

MTU vs TCP MSS (practical link)

For typical IPv4 + TCP without options: MSS ≈ MTU − 40 (20 B IP + 20 B TCP). At MTU 1500, MSS ≈ 1460; at 1492 ≈ 1452; at 1400 ≈ 1360. When a VPN lowers the effective MTU, clamping MSS on the firewall/router often fixes “page never finishes loading”, because SYN/ACK negotiates a smaller segment and avoids fragmentation.

  • Pick the target MTU first (e.g. 1400 on VPN).
  • Set MSS clamp ≈ MTU − 40 (IPv4) or MTU − 60 (IPv6 + TCP).
  • Confirm with the calculator that a typical app packet (e.g. 1500) would fragment at the new MTU — that is your signal that clamp/PMTUD is required.