MTU Test
Free Online MTU Size Checker Tool
Use our free MTU test to check your MTU size instantly. This online MTU checker detects your Maximum Transmission Unit, calculates your MSS, and checks for packet fragmentation that causes slow speeds, VPN disconnects, and gaming lag.
Quick Answer: What Is an MTU Test?
An MTU test checks your network's Maximum Transmission Unit — the largest packet size your connection can send without fragmentation. The standard Ethernet MTU is 1500 bytes. A wrong MTU size causes slow internet, VPN drops, and lag. Use this free MTU checker to instantly find your MTU size, MSS value, and optimize your connection for VPN, gaming, and everyday browsing.
Click to instantly detect your MTU size, MSS, and fragmentation status. No download needed.
Running MTU Test...
Sending test packets to detect MTU size...
MTU Test Results
Your Network Configuration
MTU Size
1500
MSS Value
1460
Connection Type
Standard Ethernet
IEEE 802.3 compliant
Recommended VPN MTU
1400 bytes
OpenVPN / WireGuard optimized
Efficiency Rating
Optimal
No overhead detected
Test Summary
Your MTU test is complete. MTU size detected: 1500 bytes (Standard Ethernet). MSS: 1460 bytes. No packet fragmentation detected. If you use a VPN, set your tunnel MTU to 1400 to prevent fragmentation.
Robert Harrison
OSINT & Network Utility Expert
Robert specializes in network diagnostics, MTU optimization, and DNS management. With years of experience in packet analysis and VPN troubleshooting, he helps users understand how to test and configure MTU for maximum network performance.
View All Articles by Robert HarrisonWhat Is an MTU Test and Why Does MTU Size Matter?
An MTU test checks your network's Maximum Transmission Unit — the largest packet of data your connection can send in one piece without breaking it apart. If your MTU size is wrong, your internet slows down, your VPN drops, and online games lag. This free MTU checker tool detects your exact MTU value, MSS (Maximum Segment Size), and tells you if packet fragmentation is hurting your speed.
Think of data like a delivery truck driving through tunnels. Each tunnel has a height limit. If the truck is too tall, it must unload and make two trips. That is exactly what happens when your data packets are bigger than your network MTU. Your router breaks them into smaller pieces — called packet fragmentation. It wastes bandwidth, increases latency, and slows everything down.
The standard MTU for most Ethernet networks is 1500 bytes. But depending on your ISP, connection type, and whether you use a VPN, your actual MTU could be lower. Running an MTU test online helps you find your MTU size so you can fix speed problems at the source.
Key Fact: According to Wikipedia's Maximum Transmission Unit documentation, the 1500-byte Ethernet MTU has been the standard since the original IEEE 802.3 specification from the early 1980s. Even on today's 100 Gbps networks, this same limit applies.
How Does the MTU Test Work? A Step-by-Step Guide
Our MTU tester sends packets of different sizes to find the largest one that passes through your connection without being split. This process is similar to Path MTU Discovery (PMTUD), which is how your computer naturally detects the MTU of every hop between you and a server.
Step 1: Send a Full-Size Packet
The tool sends a 1500-byte packet with the "Don't Fragment" (DF) bit enabled. If it goes through, your MTU is 1500 — the standard for Ethernet.
Step 2: Detect the Fragmentation Threshold
If the full-size packet fails, the tool lowers the size and retries until it finds the largest packet that fits without fragmentation. This is the same binary search that PMTUD uses.
Step 3: Calculate Your MSS Value
Your MSS equals your MTU minus 40 bytes of headers (20 for the IP header + 20 for the TCP header). With MTU 1500, your MSS is 1460 bytes.
Step 4: Identify Your Connection Type
MTU 1500 means standard Ethernet. MTU 1492 indicates a PPPoE connection. Lower values suggest VPN tunneling or other protocol overhead reducing your usable packet size.
Pro Tip: Run this MTU test both with and without your VPN connected. Compare results to see exactly how much overhead your VPN adds. Then use our Internet Speed Test to measure the real-world bandwidth impact.
MTU vs MSS: Understanding the Key Difference
Many people confuse MTU with MSS, but they measure different things. MTU (Maximum Transmission Unit) is the total size of a network packet, including all headers. MSS (Maximum Segment Size) is just the data payload — the actual information you send, without the headers.
The MTU-to-MSS Conversion Formula
- IPv4: MSS = MTU (1500) minus IP Header (20 bytes) minus TCP Header (20 bytes) = 1460 bytes
- IPv6: MSS = MTU (1500) minus IPv6 Header (40 bytes) minus TCP Header (20 bytes) = 1440 bytes
- VPN (OpenVPN over UDP): MSS = MTU (1500) minus IP (20) minus UDP (8) minus OpenVPN (32-60) = approximately 1380-1412 bytes
The IPv6 header is 20 bytes larger than IPv4, which means you get less usable payload on IPv6 connections. Our IPv6 Address Expansion Tool helps you understand IPv6 formatting. To see how IP addresses work at the binary level, use our IP to Binary Converter.
What Causes Packet Fragmentation and How to Fix It
Packet fragmentation happens when a data packet is larger than the MTU of any router or device on its path. Your router breaks it into two or more smaller packets. Each piece gets its own header, travels separately, and must be reassembled at the destination. This wastes CPU power, adds latency, and increases the chance of packet loss.
Common Causes of Packet Fragmentation
- PPPoE connections: DSL and some fiber providers use PPPoE, adding 8 bytes of overhead. Effective MTU drops from 1500 to 1492. This is the best MTU for PPPoE connections.
- VPN tunnels: OpenVPN adds 40-60 bytes of encryption overhead. WireGuard adds 60-80 bytes. This is why your VPN may feel slow.
- Double NAT: If you have two routers (a modem-router combo plus your own), each one may add overhead that reduces effective MTU.
- VLAN tagging: Enterprise networks using VLANs add 4-22 bytes per packet for 802.1Q tags.
The ICMP Black Hole Problem
The worst outcome is an ICMP black hole. This happens when a router blocks ICMP "Fragmentation Needed" messages. Your computer never learns that packets are too big, so data simply disappears. Websites half-load, VPN tunnels stall, and downloads freeze. To check if your IP is associated with a cloud provider that may enforce strict MTU rules, use our Cloud Provider IP Detection Tool.
Warning: Many firewalls block all ICMP traffic, which breaks Path MTU Discovery. If you manage a firewall, always allow ICMP Type 3 Code 4 (Destination Unreachable, Fragmentation Needed). This single rule prevents most MTU-related connection failures.
Best MTU Settings for VPN: OpenVPN, WireGuard, and IPSec
VPN connections are the number one cause of MTU problems for regular users. Every VPN protocol adds encryption headers to your data. If you do not lower your MTU to account for this overhead, you get slow connections, random disconnects, and websites that refuse to load. This is why your VPN may feel slow.
Recommended VPN MTU Values by Protocol
| VPN Protocol | Overhead Added | Recommended MTU | Resulting MSS |
|---|---|---|---|
| OpenVPN (UDP) | 48-60 bytes | 1400 | 1360 |
| WireGuard | 60-80 bytes | 1420 | 1380 |
| IPSec (ESP/AES-256) | 50-73 bytes | 1400-1438 | 1360-1398 |
| L2TP/IPSec | 70-100 bytes | 1400 | 1360 |
| PPTP | 40-50 bytes | 1440 | 1400 |
After adjusting your VPN MTU, verify that your VPN is not leaking your real IP address. Run our WebRTC Leak Detection Test and Browser Privacy Leak Checker to confirm full protection.
Best MTU Size for Gaming: Reduce Ping and Lag Spikes
Gamers are extremely sensitive to packet fragmentation. Even a few milliseconds of extra latency can mean the difference between winning and losing in fast-paced games. When your MTU is too high, every packet gets split, reassembled, and delayed — causing rubber-banding and lag spikes.
Recommended Gaming MTU by Connection Type
- Wired Ethernet: Keep MTU at 1500 bytes — the standard optimal setting for lowest latency
- Wi-Fi gaming: Try 1472 bytes — smaller packets handle wireless interference better
- VPN gaming: Set MTU to 1400 bytes — accounts for encryption overhead without fragmentation
- PPPoE DSL: Use 1492 bytes — compensates for the 8-byte PPPoE header overhead
Learn more about how game servers route your connection in our guide on How Game Servers Use Your IP for Regional Matchmaking. And if you want to see whether changing your IP could help reduce ping, read Does Changing Your IP Address Reduce Ping in Games?
How to Change MTU Size on Windows, Mac, Linux, and Router
Once you know your ideal MTU from the test above, here is how to change MTU on every major platform.
Change MTU on Windows 10 and Windows 11
# Step 1: Check your current MTU value
netsh interface ipv4 show subinterfaces
# Step 2: Set new MTU (replace 1400 with your optimal value)
netsh interface ipv4 set subinterface "Ethernet" mtu=1400 store=persistent
# Step 3: Verify the change was applied
netsh interface ipv4 show subinterfaces
Change MTU on macOS
# Check current MTU on macOS
ifconfig en0 | grep mtu
# Set new MTU value on macOS
sudo ifconfig en0 mtu 1400
Change MTU on Linux (Ubuntu, Debian, CentOS)
# Check current MTU on Linux
ip link show eth0
# Set new MTU temporarily on Linux
sudo ip link set dev eth0 mtu 1400
# Make it permanent on Ubuntu/Debian using netplan
sudo nano /etc/netplan/01-netcfg.yaml
How to Check and Change MTU on Your Router
Log into your router admin panel (usually at 192.168.1.1 or 192.168.0.1). Navigate to WAN Settings or Internet Settings. Find the MTU field and enter your ideal value — use 1492 for PPPoE or 1400 for VPN. Save the settings and reboot your router. If you need to identify your router's IP details, our IP Geolocation Lookup Tool can help.
How to Test MTU Manually Using the Ping Command
If you prefer to manually test MTU without an online tool, here is how to do it with the ping command on any operating system.
Windows Manual Ping MTU Test
ping -f -l 1472 8.8.8.8
# -f = Set the Don't Fragment flag
# -l = Set the packet payload size (lowercase L)
# If you see "Packet needs to be fragmented", lower the number
# Add 28 to your largest successful value to get your real MTU
macOS and Linux Manual Ping MTU Test
ping -M do -s 1472 -c 4 8.8.8.8
# -M do = Set the Don't Fragment flag on macOS/Linux
# -s = Set the packet payload size | -c = Number of pings to send
Start at 1472 (which gives MTU 1500 when you add 28 bytes for IP and ICMP headers). If fragmentation occurs, lower by 10 each time until it passes. Then fine-tune upward by 1 byte at a time. Add 28 to the final number to get your real MTU.
For deeper network troubleshooting, combine this with our Network Ping Test Tool, DNS Lookup Tool, and Open Port Scanner Tool.
Common MTU Values by Connection Type: Complete Reference
Different network technologies use different default MTU values. Here is a complete reference table showing MTU 1500 vs 1400 and all other common values:
| Connection Type | Default MTU | Notes |
|---|---|---|
| Ethernet (Standard) | 1500 | Most common for home and office networks |
| PPPoE (DSL/Fiber) | 1492 | 8 bytes consumed by PPPoE header overhead |
| Wi-Fi (802.11) | 1500 | Same as Ethernet, but wireless interference may cause issues |
| VPN (OpenVPN) | 1400 | Encryption overhead reduces usable packet space |
| VPN (WireGuard) | 1420 | More efficient encryption than OpenVPN needs less overhead |
| Jumbo Frames | 9000 | Data centers, SANs, and high-performance computing only |
| IPv6 Minimum | 1280 | Smallest MTU allowed under the IPv6 standard |
| Satellite Internet | 512-1500 | Varies by provider, orbit altitude, and weather |
If you are on Starlink satellite internet, read our detailed guide: Starlink IP Addressing and How Satellite Internet Works.
Jumbo Frames Explained: When MTU Goes Beyond 1500 to 9000
In data centers and high-performance networks, engineers use Jumbo Frames with an MTU of up to 9000 bytes. Sending one 9000-byte packet uses far less CPU than sending six 1500-byte packets. This is critical for storage area networks (SANs), NVMe/TCP, iSCSI traffic, and large-scale backups.
However, jumbo frames do not work on the public internet. Every router between you and the destination must support the same MTU. If a 9000-byte packet hits a standard Ethernet link, it gets dropped instantly because the Don't Fragment (DF) bit in the IP header prevents silent fragmentation.
If you manage servers and want to check their security alongside MTU settings, our JA3 TLS Fingerprint Analysis Tool identifies unusual handshake patterns. Also verify your certificates with our SSL Certificate Checker.
MTU and Network Security: Attack Vectors You Should Know
MTU is not just a performance setting — it is also a security concern. Attackers have used packet fragmentation and MTU manipulation in several types of network attacks:
- Teardrop Attack: Sends overlapping fragmented packets designed to crash the target operating system when it tries to reassemble them.
- Tiny Fragment Attack: Splits the TCP header across two fragments to sneak malicious traffic past firewalls that only inspect the first fragment.
- Fragmentation-Based Evasion: Crafts packets just under the MTU limit to avoid deep packet inspection (DPI) systems used by firewalls.
Testing your MTU regularly confirms your hardware handles fragmentation correctly. For additional security, use our IP Fraud Score Analysis Tool and IP Blacklist Reputation Checker to verify your IP is clean.
When Should You Lower Your MTU Below 1500?
You should lower your MTU from the default 1500 if you experience any of these common problems that indicate packet fragmentation:
- Your VPN connects but websites load very slowly or not at all
- You see frequent packet loss during speed tests
- Online games have sudden lag spikes or rubber-banding
- Large file downloads fail, stall, or restart repeatedly
- Your ISP uses PPPoE (common with DSL and some fiber connections)
- You are behind a double NAT setup with two routers
- Video calls on Zoom, Teams, or Google Meet freeze or drop
To verify your full connection health after changing MTU, run a complete audit using our Browser Information Detection Tool, HTTP Headers Analyzer, Font-Based Leak Detection Test, and HTTP Referrer Leak Test.
MTU Test: Frequently Asked Questions
What is an MTU test used for?
An MTU test is used to find your MTU size — the largest packet your network can send without fragmentation. It helps fix VPN disconnects, reduce gaming lag, solve slow internet problems, and optimize your connection for maximum speed.
What is the best MTU size for gaming?
For wired Ethernet gaming, keep MTU at 1500. On Wi-Fi, try 1472. With a VPN, use 1400. The goal is to avoid packet fragmentation that causes latency spikes and lag during gameplay.
How do I find my MTU size on Windows?
Use this free MTU test tool, or open Command Prompt and run netsh interface ipv4 show subinterfaces. For manual testing, run ping -f -l 1472 8.8.8.8 and lower the number until fragmentation stops. Add 28 to get your actual MTU.
What is the ideal MTU for VPN connections?
OpenVPN works best at MTU 1400. The optimal MTU for WireGuard is 1420. IPSec tunnels need 1400-1438. VPN encryption adds 40-80 bytes of overhead per packet, so you always need a lower MTU than the default 1500.
Does MTU affect internet speed?
Yes, MTU directly affects internet speed. A wrong MTU forces your router to fragment packets, which increases CPU load, adds latency, and wastes bandwidth on duplicate headers. Properly tuning your MTU can improve effective throughput by 5-15 percent.
What is the difference between MTU and MSS?
MTU is the total packet size including all headers. MSS is the data payload only, without headers. The formula is: MSS = MTU minus 40 bytes (20 IP header + 20 TCP header). With MTU 1500, your MSS equals 1460 bytes.
Why does my VPN keep disconnecting?
MTU mismatch is a top cause of VPN disconnects. VPN encryption makes packets larger than the path MTU, so they get silently dropped by intermediate routers. Lower your MTU to 1400 or enable MSS clamping on your router to fix this problem.
How do I check and change MTU on my router?
Log into your router admin panel at 192.168.1.1 or 192.168.0.1. Go to WAN Settings. Your current MTU is displayed there. Change it to 1492 for PPPoE or 1400 for VPN use. Save settings and reboot. Use our complete network tools directory to verify your changes.
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Test Your MTU Now
Free Online MTU Size Checker Tool
Check your MTU size, fix packet fragmentation, and optimize your connection for VPN, gaming, and everyday browsing. Our free MTU test works instantly — no download needed.