Default MTU Size for Different Network Topology

• Microsoft Windows NT Workstation versions 3.5 and 3.51
• Microsoft Windows NT Server versions 3.5 and 3.51
• Microsoft TCP/IP-32 for Windows for Workgroups versions 3.11, 3.11a, and 3.11b
• Microsoft Windows NT Server version 4.0
• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows 95

MTU (Maximum Transfer Unit) specifies the maximum transmission unit size of an interface. Each interface used by TCP/IP may have a different MTU value specified. The MTU is usually determined through negotiation with the lower driver, using that lower driver value. However, that value may be overridden.

Each media type has a maximum frame size that cannot be exceeded. The link layer is responsible for discovering this MTU and reporting it to the protocols above. NDIS drivers may be queried for the local MTU by the protocol stack. Knowledge of the MTU for an interface is used by upper layer protocols such as TCP, which optimizes packet sizes for each medium automatically.

If a network interface card (NIC) driver such as an ATM driver uses LAN emulation mode, it may report that it has an MTU higher than what is expected for that media type. For instance, it may emulate Ethernet but report an MTU of 9180 bytes. Windows NT accepts and uses the MTU size reported by the adapter even when it exceeds the normal MTU for a given media type.

Below is a list of Default MTU size for different media.

   Network                  MTU(Bytes)
   -----------------------------------
   16 Mbit/Sec Token Ring    17914
   4 Mbits/Sec Token Ring     4464
   FDDI                       4352
   Ethernet                   1500
   IEEE 802.3/802.2           1492
   X.25                        576