How to install IFTOP the right way?
/usr/local/sbin is not in your user's PATH variable. The most simple method is adding the path /usr/local/sbin to the file /private/etc/paths:

sudo nano /private/etc/paths


Modify:

/usr/local/bin
/usr/bin
/bin
/usr/sbin
/sbin


to

/usr/local/bin
/usr/bin
/bin
/usr/local/sbin
/usr/sbin
/sbin


and save the file to disk with ctrlO and  ctrlX. Then exit the shell or restart Terminal. This will change the PATH for all users.

If you want to restrict it to your user create and/or edit the file ~/.bash_profile with nano and add the line:

export PATH="/usr/local/sbin:$PATH"


Save the file to disk with ctrlO and  ctrlX. Then exit the shell or restart Terminal. This will change the PATH for your user only.



To install iftop with brew from scratch in High Sierra with prerequisites (Xcode | Xcode Command Line Tools) already installed , open Terminal.app and enter:

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew analytics off #optional
brew doctor #optional
brew install iftop


and add the /usr/local/sbin to the PATH as described earlier.