Complex sentence: IDP packets were up to 576 bytes long, including the 30 byte IDP header, but smaller than IP which required all hosts to support at least 576, but supports packets of up to 65K bytes.
Split into simpler sentences: IDP packets were up to 576 bytes long, including the 30 byte IDP header. In comparison, IP required all hosts to support at least 576, but supports packets of up to 65K bytes.