Find the ethernet cable length you need.
Cable runs are always longer than the straight-line distance because the cable follows walls, ceilings and trunking rather than the shortest path. Adding the vertical rise, a routing allowance for obstacles, and service slack at both ends gives a realistic figure, then rounding up to a stocked length avoids a run that is short by a metre. The margin against the 100 m TIA/ISO channel limit is reported because exceeding it degrades a gigabit link.
Routed cable length
Routed = (horizontal + vertical) x (1 + detour allowance) + 2 x slack per end; purchase = routed rounded up to the next 5 m; margin = 100 - routed
The TIA-568 and ISO 11801 standards specify a 100 m channel: up to 90 m of fixed horizontal cabling plus 10 m of patch leads. Beyond that, signal attenuation and delay skew put a gigabit link outside specification.
About 1-1.5 m at each end is normal practice — enough to re-terminate a damaged connector or move equipment within the rack or room without pulling a new run.
Not immediately. A 110-120 m run of good Cat6 often negotiates gigabit and appears to work, but with reduced error margin and intermittent renegotiation. For a reliable link beyond 100 m, use a switch in between or fibre.