HLD and LLD are 2 terms used commonly in Network Design, Operation and implementation.While HLD refers to High Level Design , LLD is abbreviation for Low Level Design.
HLD is created initially during the Design journey of a Solution. It provides a high level view of overall System setup describing relationship of various systems and functions which combine to provide the expected solution. LLD is created on the footsteps of HLD and provides detailed and in depth information of the HLD.Some terms related to HLD include Zoning,Traffic flow and high level connectivity across various elements of solution , while LLD is more related to terms like VLAN, IP address, port numbering etc.
Further difference between HLD and LLD is detailed in below table –