STB is Set Top Box abbreviation.Its used in Triple play solution .It has certain ports (RJ11 for telephone connectivity ,RJ45 for Internet ,and TV out for television services) all these signals travel over either FIber or Coax cable to the 3 headends at service provider's core network.
STB ip address would be the ip address assigned to SET TOP BOx device in a purely IP network to the service provider providing triple play services (phone,internet n TV ).
An IP set top box is an electronic device that adapts IP television data into a format that is accessible by the end user. The output of an IP set top box can be a television RF channel (e.g. channel 3), video and audio signals or digital video signals. IP set top boxes are commonly located in a customer's home to allow the reception of IP video signals on a television or computer