How far into the future in networkdesign

Depends on the dynamics of the company.

Generally, 6 months to a year out forecasts are reasonable.

2 to 3 years are reasonable.

Beyond 3 to 5 years, you are wasting your time. If for no other reason, than the progression of computer and network change.

Chas

This will depend a lot upon how well the organisation is growing. A simple rule of thumb that I was given at college was:

If the organisation has over 1,000 host allow for a 25% increase in demands over current needs, as a minimum.

If the organsiation is less than a 1,000 hosts allow for double the current and anticipated demands.

If the organisation is less than 250 hosts allow for four times the current and anticipated demand.

If known to be growing fast allow more than the above.

Once you design your network to handle these new loads you know it will be viable for some time to come. When building the new network you should incorporate wiring to suit the full design but only install hardware as needed to meet current and client anticipated needs plus 10%. then expand hardware etc as required to meet further demands. You may need a 100 port router in the long term design but a 24 port may do for today, install the 24 port and note the need for more later, etc.