What is the maximum distance that the 40GBASE-LR4 Ethernet standard can span?

550 m is the maximum distance for 40GBASE-SR4, which uses parallel multimode fiber and is designed for short-range intra-data-center links.

40GBASE-LR4 is defined in IEEE 802.3ba and transmits 40 Gbps by multiplexing four 10 Gbps optical channels at distinct wavelengths (LAN WDM) over single-mode fiber. The standard specifies a maximum link distance of 10 km, making it well suited for campus and metro Ethernet connections that exceed data center short-reach limitations.

40 km corresponds to 40GBASE-ER4, an extended-reach single-mode variant that uses cooled DFB lasers; the LR4 specification is limited to 10 km.

80 km is associated with certain 100G extended-reach optics such as 100GBASE-ZR or amplified links, not with the 40GBASE-LR4 standard.