In real mode or V86 mode it is possible to form a linear address as large as 10FFEFh (FFFF0h + FFFFh), or roughly 1MB + 64K. On the 8086, which has only 20 address lines, linear addresses greater than 1MB wrap to first 64K of physical memory. Incredibly, some software designed for the 8086 relies on this behavior.
The largest address that can be formed in V86 mode is FFFF:FFFF, which is equivalent to the linear address 10FFEFh (FFFF0h + FFFFh).