Security and Technology Ramblings…

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Sorry, just putting this here so I don’t forget it.

This document describes the procedure used in order to calculate the hexadecimal value for the time offset DHCP Option 2 when DHCP pools are configured in Cisco routers. This option is particularly important in cable environments.

The DHCP configurations option is available in many Cisco platforms, in particular in uBR7200, and all the other uBRs. Cisco routers that run a Cisco IOS® release of 12.0(1)T or later have the ability to act as DHCP servers.

When you use the Cisco IOS DHCP server, the time offset value for a particular time zone is specified as an unsigned 32 bit hexadecimal value.

There is a common misconception about the difference between the time offset and Network Time Protocol (NTP). NTP is used by machines on the Internet in order to synchronize with the UTC timescale. The acronym UTC is an English-French mixture for Coordinated Universal Time (or its equivalent Temps Universel Coordonne) in order to make it language independent. UTC is more precise than GMT because UTC measures time based on an atomic clock. Time offset is based on Greenwich Mean Time (GMT) which defines a second as 1/86400 of a day, which is not 100 percent accurate.

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