Optimize network performance by calculating effective MTU (Maximum Transmission Unit) and TCP MSS based on your connection type. Supports Ethernet, PPPoE, VPN, and more.
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Maximum Transmission Unit (MTU) determines the largest packet size that can be transmitted without fragmentation. This calculator helps you determine the effective MTU and TCP Maximum Segment Size (MSS) based on your connection type and protocol overhead, ensuring optimal network performance.
Packets larger than the MTU must be fragmented, increasing latency and reducing throughput. Correctly sized packets transmit more efficiently.
VPN tunnels add protocol overhead, reducing effective MTU. Knowing the correct MTU prevents connection issues and slow speeds.
Many connectivity problems stem from MTU mismatches. Path MTU discovery failures cause black hole routing where packets disappear.
ISPs and network equipment need proper MTU settings. PPPoE connections commonly require MTU adjustments for reliable operation.
MTU (Maximum Transmission Unit) is the largest packet size that can traverse a network path without fragmentation. Oversized packets are either fragmented (causing overhead) or dropped (causing retransmissions). Correct MTU ensures efficient data transfer.
TCP MSS (Maximum Segment Size) is the largest amount of TCP data that can fit in a single packet. It's calculated as MTU minus 40 bytes (20 bytes for IP header + 20 bytes for TCP header). This value is negotiated during the TCP handshake.
PPPoE (Point-to-Point Protocol over Ethernet) adds an 8-byte header to every packet. With a 1500-byte Ethernet MTU, PPPoE reduces effective MTU to 1492 bytes. Many DSL and fiber connections use PPPoE for authentication.
VPN overhead varies by protocol: WireGuard adds ~60 bytes, OpenVPN ~69 bytes, and IPSec VPN ~73 bytes. Subtract this from your base MTU. For PPPoE + VPN, subtract both overheads. Using 1400 is often a safe default for VPN connections.
Use the ping command with Don't Fragment flag: 'ping -f -l [size] [host]' (Windows) or 'ping -M do -s [size] [host]' (Linux). Start at 1500 and reduce by 10 until packets succeed. Add 28 bytes (IP+ICMP headers) to get your MTU.
Path MTU Discovery (PMTUD) automatically determines the minimum MTU along a network path by sending packets with the Don't Fragment flag. When a packet is too large, routers send ICMP 'Fragmentation Needed' messages back. Some firewalls block these, causing black hole issues.