preface

Dynamic Host Configuration Protocol DHCP (Dynamic Host Configuration Protocol) is a network management protocol that is used to centrally manage and configure the user’s IP address dynamically. DHCP became the standard protocol in October 1993, formerly known as the BOOTP protocol. The DHCP protocol, defined by RFC 2131, uses a client/server communication pattern in which a configuration request is made by the DHCP Client to the server (DHCP Server), which dynamically assigns ip addresses, subnet masks, default gateway addresses, domain name server (DNS) addresses, and other related configuration parameters for each device on the network so that it can communicate with other IP networks.

1. WHY USE DHCP?

In an IP network, each Internet-connected device needs to be assigned a unique IP address. DHCP enables network administrators to monitor and assign IP addresses from a central node. When a computer moves to another location on the network, it automatically receives a new IP address. The automated assignment of IP addresses enabled by DHCP not only reduces the time to configure and deploy devices, but also reduces the likelihood of configuration errors. In addition, the DHCP server can manage the configuration information of multiple network segments, and when the configuration of a network segment changes, the administrator only needs to update the relevant configuration on the DHCP server, which realizes centralized management.

OVERALL, DHCP OFFERS THE FOLLOWING ADVANTAGES:

  • ACCURATE IP CONFIGURATION: IP ADDRESS CONFIGURATION PARAMETERS MUST BE ACCURATE AND PRONE TO ERRORS WHEN DEALING WITH INPUTS SUCH AS “192.168.XXX.XXX”. IN ADDITION, TYPOGRAPHICAL ERRORS ARE OFTEN DIFFICULT TO RESOLVE, AND USING A DHCP SERVER MINIMIZES THIS RISK.
  • REDUCE IP ADDRESS CONFLICTS: EACH CONNECTED DEVICE MUST HAVE AN IP ADDRESS. HOWEVER, EACH ADDRESS CAN ONLY BE USED ONCE, AND DUPLICATE ADDRESSES WILL RESULT IN CONFLICTS WHERE ONE OR BOTH DEVICES CANNOT BE CONNECTED. THIS CAN HAPPEN WHEN ASSIGNING ADDRESSES MANUALLY, ESPECIALLY WHEN THERE ARE A LARGE NUMBER OF ENDPOINTS THAT ARE ONLY CONNECTED PERIODICALLY, SUCH AS MOBILE DEVICES. THE USE OF DHCP ENSURES THAT EACH ADDRESS IS USED ONLY ONCE.
  • AUTOMATION OF IP ADDRESS MANAGEMENT: WITHOUT DHCP, NETWORK ADMINISTRATORS WILL NEED TO MANUALLY ASSIGN AND REVOKE ADDRESSES. TRACKING WHICH DEVICE HAS WHAT ADDRESS CAN BE FUTILE, AS IT IS ALMOST IMPOSSIBLE TO UNDERSTAND WHEN A DEVICE NEEDS TO ACCESS THE NETWORK AND WHEN IT NEEDS TO LEAVE THE NETWORK. DHCP ALLOWS IT TO BE AUTOMATED AND CENTRALIZED, SO NETWORK PROFESSIONALS CAN MANAGE ALL LOCATIONS FROM ONE LOCATION.
  • EFFICIENT CHANGE MANAGEMENT: THE USE OF DHCP MAKES IT EASY TO CHANGE ADDRESSES, RANGES, OR ENDPOINTS. FOR EXAMPLE, AN ORGANIZATION MIGHT WANT TO CHANGE ITS IP ADDRESSING SCHEME FROM ONE RANGE TO ANOTHER. THE DHCP SERVER IS CONFIGURED WITH NEW INFORMATION THAT IS PROPAGATED TO THE NEW ENDPOINT. SIMILARLY, IF YOU UPGRADE AND REPLACE A NETWORK DEVICE, YOU DO NOT NEED A NETWORK CONFIGURATION.

2. HOW DOES DHCP WORK?

THE DHCP PROTOCOL USES UDP AS THE TRANSPORT PROTOCOL, WHERE THE DHCP CLIENT SENDS A REQUEST MESSAGE TO PORT 68 OF THE DHCP SERVER, AND THE DHCP SERVER RESPONDS WITH A REPLY MESSAGE TO PORT 67 OF THE DHCP CLIENT.

ONLY DHCP SERVERS THAT ARE ON THE SAME NETWORK SEGMENT AS THE DHCP CLIENT CAN RECEIVE DHCP DISCOVER MESSAGES BROADCAST BY THE DHCP CLIENT. WHEN A DHCP CLIENT AND A DHCP SERVER ARE NOT ON THE SAME NETWORK SEGMENT, YOU MUST DEPLOY DHCP RELAYS TO FORWARD DHCP PACKETS BETWEEN THE DHCP CLIENT AND THE DHCP SERVER. DHCP RELAYS LOOK TO DHCP CLIENTS LIKE DHCP SERVERS; TO DHCP SERVERS, DHCP RELAYS LOOK LIKE DHCP CLIENTS.

2.1 HOW DHCP CLIENTS ACCESS THE NETWORK FOR THE FIRST TIME IN A NO-TRUNK SCENARIO

AS SHOWN IN THE FOLLOWING FIGURE, IN THE CASE WHERE DHCP RELAY IS NOT DEPLOYED, THE PACKET INTERACTION PROCESS BETWEEN THE DHCP CLIENT AND THE DHCP SERVER THAT ACCESSES THE NETWORK FOR THE FIRST TIME IS CALLED THE DHCP MESSAGE FOUR-STEP INTERACTION.

SCHEMATIC DIAGRAM OF THE PACKET INTERACTION OF THE DHCP CLIENT ACCESSING THE NETWORK FOR THE FIRST TIME IN A NO-RELAY SCENARIO

step 1: discovery phase

The DHCP client accessing the network for the first time does not know the IP address of the DHCP server, and in order to learn the IP address of the DHCP server, the DHCP client broadcasts a DHCP DISCOVER message (the destination IP address is 255.255.255.255) to all devices in the same network segment (including the DHCP server or trunk). The DHCP DISCOVER message carries information such as the mac address of the client (chaddr field), the parameter list option that needs to be requested (Option55), and the broadcast flag bit (flags field).

step 2: provision phase

DHCP SERVERS THAT ARE IN THE SAME NETWORK SEGMENT AS THE DHCP CLIENT RECEIVE DHCP DISCOVER PACKETS, AND THE DHCP SERVER SELECTS THE ADDRESS POOL IN THE SAME NETWORK SEGMENT AS THE IP ADDRESS OF THE INTERFACE THAT RECEIVES THE DHCP DISCOVER PACKET, SELECTS AN AVAILABLE IP ADDRESS FROM IT, AND THEN SENDS THE DHCP PACKET TO THE DHCP CLIENT VIA DHCP OFFER.

TYPICALLY, THE DHCP SERVER SPECIFIES THE LEASE PERIOD OF THE IP ADDRESS IN THE ADDRESS POOL, AND IF THE DHCP CLIENT SENDS A DHCP DISCOVER MESSAGE WITH THE EXPECTED LEASE, THE SERVER COMPARES THE EXPECTED LEASE REQUESTED BY THE CLIENT TO ITS SPECIFIED LEASE AND SELECTS THE SHORTER LEASE TO ASSIGN TO THE CLIENT. THE ORDER IN WHICH THE DHCP SERVER ASSIGNS IP ADDRESSES TO CLIENTS IN THE ADDRESS POOL IS AS FOLLOWS:

  • AN IP ADDRESS THAT HAS BEEN CONFIGURED ON THE DHCP SERVER TO BE STATICALLY BOUND TO THE CLIENT MAC ADDRESS.
  • The address specified in Option50 (Request IP Address option) in the DHCP DISCOVER message sent by the client.
  • Look for the IP address with the status of “Expired” within the address pool, that is, the IP address that was once assigned to the client for an extended lease.
  • Randomly look for an IP address with an “Idle” status within the address pool.
  • If no IP addresses are found available for assignment, the address pool automatically reclaims IP addresses that have exceeded the lease period (“Expired” state) and then the IP addresses that are in conflicting states (“Conflict”). After reclamation, if an available IP address is found, it is assigned; otherwise, the DHCP client waits for the answer to time out and resends the DHCP DISCOVER message to request the IP address.

THE APPLIANCE SUPPORTS EXCLUDING CERTAIN IP ADDRESSES FROM THE ADDRESS POOL THAT CANNOT BE ASSIGNED THROUGH THE DHCP MECHANISM. FOR EXAMPLE, IF THE CLIENT IS IN A NETWORK SEGMENT THAT HAS MANUALLY CONFIGURED A DNS SERVER WITH AN ADDRESS OF 192.168.1.100/24, THE ADDRESS POOL CONFIGURED ON THE DHCP SERVER WITH A NETWORK SEGMENT OF 192.168.1.0/24 NEEDS TO EXCLUDE THE IP ADDRESS OF 192.168.1.100, WHICH CANNOT BE ASSIGNED THROUGH DHCP, OTHERWISE, IT WILL CAUSE AN ADDRESS CONFLICT.

IN ORDER TO PREVENT THE ASSIGNED IP ADDRESS FROM CONFLICTING WITH THE IP ADDRESSES OF OTHER CLIENTS IN THE NETWORK, THE DHCP SERVER PROBES THE ASSIGNED IP ADDRESS BY SENDING AN ICMP ECHO REQUEST MESSAGE WITH THE SOURCE ADDRESS BEING THE DHCP SERVER IP ADDRESS AND THE DESTINATION ADDRESS BEING THE PRE-ASSIGNED IP ADDRESS BEFORE SENDING THE DHCP OFFER PACKET. IF NO REPLY PACKET IS RECEIVED WITHIN THE SPECIFIED TIME, IT MEANS THAT NO CLIENT IN THE NETWORK USES THIS IP ADDRESS AND CAN BE ASSIGNED TO THE CLIENT; IF THE REPLY PACKET IS RECEIVED WITHIN THE SPECIFIED TIME, INDICATING THAT A CLIENT WITH THIS IP ADDRESS ALREADY EXISTS IN THE NETWORK, THIS ADDRESS IS LISTED AS A CONFLICTING ADDRESS, AND THEN WAITS FOR THE DHCP DISCOVER PACKET TO BE RECEIVED AGAIN AND RESELECTS THE AVAILABLE IP ADDRESSES ACCORDING TO THE ORDER OF PRECEDENCE FOR SELECTING IP ADDRESSES DESCRIBED EARLIER.

THE IP ADDRESS ASSIGNED TO THE CLIENT BY THE DHCP SERVER AT THIS STAGE IS NOT NECESSARILY THE IP ADDRESS THAT IS FINALIZED TO BE USED, BECAUSE THE DHCP OFFER MESSAGE SENT TO THE CLIENT WAITS 16 SECONDS AND THEN IF NO RESPONSE IS RECEIVED FROM THE CLIENT, THE ADDRESS CAN CONTINUE TO BE ASSIGNED TO OTHER CLIENTS. THE FOLLOWING SELECTION AND CONFIRMATION PHASES ARE REQUIRED TO FINALIZE THE IP ADDRESSES THAT CLIENTS CAN USE.

step 3: select the stage

If there are multiple DHCP servers responding to a DHCP OFFER message to a DHCP client, the DHCP client typically receives only the first received DHCP OFFER packet and then sends a DHCP REQUEST packet in broadcast mode that contains the DHCP server identifier (that is, Option54) and the client IP address (that is, Option50) that the client wants to select, populating the received DHCP OFFER message with yiaddr IP Address of the field).

THE DHCP CLIENT BROADCAST SENDS A DHCP REQUEST MESSAGE TO NOTIFY ALL DHCP SERVERS THAT IT WILL SELECT THE IP ADDRESS PROVIDED BY ONE DHCP SERVER, AND THE OTHER DHCP SERVER CAN REASSIGN THE IP ADDRESS THAT WAS PREVIOUSLY ASSIGNED TO THE CLIENT TO ANOTHER CLIENT.

step 4: confirmation phase

When the DHCP server receives a DHCP REQUEST message sent by a DHCP client, the DHCP server responds to the DHCP ACK packet, indicating that the IP address requested in the DHCP REQUEST packet (filled by Option50) is assigned to the client for use.

WHEN A DHCP CLIENT RECEIVES A DHCP ACK PACKET, IT BROADCASTS AND SENDS A FREE ARP PACKET TO DETECT WHETHER OTHER ENDPOINTS IN THE NETWORK SEGMENT USE THE IP ADDRESS ASSIGNED BY THE SERVER, AND IF NO RESPONSE IS RECEIVED WITHIN THE SPECIFIED TIME, THE CLIENT CAN USE THIS ADDRESS. IF A RESPONSE IS RECEIVED INDICATING THAT ANOTHER TERMINAL IS USING THIS ADDRESS, THE CLIENT SENDS A DHCP DECLINE MESSAGE TO THE SERVER AND REQUESTS THE IP ADDRESS AGAIN FROM THE SERVER, AND THE SERVER LISTS THE ADDRESS AS A CONFLICTING ADDRESS. WHEN THE SERVER HAS NO IDLE ADDRESS TO ASSIGN, SELECT THE CONFLICTING ADDRESS TO ALLOCATE TO MINIMIZE THE ADDRESS CONFLICTS THAT ARE ALLOCATED.

When a DHCP server receives a DHCP REQUEST message sent by a DHCP client, if the DHCP server is unable to assign the IP address that is populated by Option50 in the DHCP REQUEST packet for some reason, such as a negotiation error or because the server has assigned this address to another client due to too slow to send a REQUEST, the DHCP NAK message is sent as an answer, informing the DHCP client that the IP address cannot be assigned. The DHCP client needs to resend the DHCP DISCOVER message to request a new IP address.

2.2 HOW DHCP CLIENTS ACCESS THE NETWORK FOR THE FIRST TIME WHEN THERE IS A RELAY SCENARIO

IN SCENARIOS WITH DHCP RELAY, DHCP CLIENTS AND DHCP SERVERS THAT ACCESS THE NETWORK FOR THE FIRST TIME WORK THE SAME WAY THAT DHCP CLIENTS ACCESS THE NETWORK FOR THE FIRST TIME IN A NO-RELAY SCENARIO. THE MAIN DIFFERENCE IS THAT DHCP RELAY FORWARDS DHCP PACKETS BETWEEN THE DHCP SERVER AND THE DHCP CLIENT TO ENSURE THAT THE DHCP SERVER AND DHCP CLIENT CAN INTERACT NORMALLY. THE FOLLOWING IS ONLY DESCRIBED FOR HOW DHCP RELAY WORKS.

AS SHOWN IN THE FOLLOWING FIGURE, IN THE SCENARIO WHERE DHCP RELAY IS DEPLOYED, THE PACKET INTERACTION PROCESS BETWEEN THE DHCP CLIENT AND THE DHCP SERVER IS FIRST ACCESSED TO THE NETWORK.

SCHEMATIC DIAGRAM OF THE PACKET INTERACTION OF THE DHCP CLIENT ACCESSING THE NETWORK FOR THE FIRST TIME WHEN THERE IS A RELAY SCENARIO

step <>: discovery phase

AFTER THE DHCP RELAY RECEIVES THE DHCP DISCOVER MESSAGE SENT BY THE DHCP CLIENT BROADCAST, IT PROCESSES IT AS FOLLOWS:

  • Check the hops field in the DHCP packet, discard the DHCP packet if greater than 16, otherwise, add the hops field to 1 (indicating a DHCP relay) and continue with the following operation.
  • Check the giaddr field in the DHCP packet. If 0, set the giaddr field to the IP address of the interface that receives DHCP DISCOVER packets. If it is not 0, the field is not modified and continue with the following operations.
  • CHANGE THE DESTINATION IP ADDRESS OF THE DHCP PACKET TO THE IP ADDRESS OF THE DHCP SERVER OR NEXT-HOP RELAY, CHANGE THE SOURCE ADDRESS TO THE INTERFACE ADDRESS OF THE RELAY-CONNECTING CLIENT, AND SEND THE DHCP PACKET UNICAST TO THE DHCP SERVER OR NEXT-HOP RELAY THROUGH ROUTING FORWARDING.

IF THERE ARE MULTIPLE DHCP RELAYS BETWEEN A DHCP CLIENT AND A DHCP SERVER, THE SUBSEQUENT RELAYS RECEIVE DHCP DISCOVER PACKETS IN THE SAME PROCESS AS DESCRIBED EARLIER.

step <>: provision phase

After the DHCP server receives the DHCP DISCOVER packet, selects the address pool that is the same network segment as the giaddr field in the packet, assigns parameters such as IP address to the client, and then sends a DHCP OFFER packet to the DHCP relay unicast identified by the giaddr field.

WHEN A DHCP RELAY RECEIVES A DHCP OFFER MESSAGE, IT PROCESSES IT AS FOLLOWS:

  • check the giaddr field in the message, discard the message if it is not the address of the interface, otherwise, continue with the following operation.
  • DHCP RELAY CHECKS THE BROADCAST FLAG BIT OF A PACKET. IF THE BROADCAST FLAG IS 1, THE DHCP OFFER MESSAGE BROADCAST IS SENT TO THE DHCP CLIENT; OTHERWISE, THE DHCP OFFER PACKET UNICAST IS SENT TO THE DHCP CLIENT.

第三步:选择阶段

THE PROCESSING OF A DHCP REQUEST MESSAGE RECEIVED BY THE RELAY FROM THE CLIENT IS THE SAME AS THE SELECTION PHASE IN A NO-RELAY SCENARIO.

step <>: confirmation phase

THE PROCESSING OF DHCP ACK PACKETS RECEIVED BY THE RELAY FROM THE SERVER IS THE SAME AS THE ACKNOWLEDGMENT PHASE IN THE NO-RELAY SCENARIO.

2.3 HOW DHCP CLIENTS REUSE ADDRESSES THAT THEY HAVE USED IN THE PAST WORK

DHCP CLIENTS CAN REUSE ADDRESSES THAT THEY HAVE USED IN THE PAST WHEN THEY ARE NOT ACCESSING THE NETWORK FOR THE FIRST TIME. AS SHOWN IN THE FOLLOWING FIGURE, A DHCP CLIENT INTERACTS WITH A DHCP SERVER WITH A DHCP MESSAGE TO REGAIN NETWORK PARAMETERS SUCH AS THE IP ADDRESS THAT WAS PREVIOUSLY USED, A PROCESS KNOWN AS TWO-STEP INTERACTION.

*DHCP CLIENTS REUSE MESSAGE INTERACTIONS FOR IP ADDRESSES THAT HAVE BEEN USED*

step 1: select the stage

The client broadcast sends a DHCP REQUEST message containing the previously assigned IP address, and the Option50 (Ip Address Option for Request) field in the message is filled in with the IP address that was previously used.

step 2: confirmation phase

AFTER THE DHCP SERVER RECEIVES THE DHCP REQUEST PACKET, IT LOOKS FOR A CORRESPONDING LEASE RECORD BASED ON THE MAC ADDRESS CARRIED IN THE DHCP REQUEST PACKET, AND IF SO, RETURNS A DHCP ACK PACKET INFORMING THE DHCP CLIENT THAT IT CAN CONTINUE TO USE THE IP ADDRESS. OTHERWISE, REMAIN SILENT AND WAIT FOR THE CLIENT TO RESEND THE DHCP DISCOVER MESSAGE REQUESTING A NEW IP ADDRESS.

2.4 HOW DHCP CLIENT UPDATE LEASES WORK

WHEN A DHCP SERVER USES A DYNAMIC ALLOCATION MECHANISM TO ASSIGN IP ADDRESSES TO CLIENTS, THE ASSIGNED IP ADDRESSES HAVE A LEASE TERM LIMIT. DHCP CLIENTS CAN CARRY AN EXPECTED LEASE WHEN REQUESTING AN ADDRESS FROM THE SERVER, AND THE SERVER ASSIGNS ONE OF THE SHORTER LEASES TO THE CLIENT BY COMPARING THE CLIENT’S EXPECTED LEASE TO THE ADDRESS POOL WHEN ALLOCATING THE LEASE. AFTER THE LEASE EXPIRES OR THE CLIENT GOES OFFLINE TO RELEASE THE ADDRESS, THE SERVER RECLAIMS THE IP ADDRESS AND THE RECLAIMED IP ADDRESS CAN CONTINUE TO BE ASSIGNED TO OTHER CLIENTS. THIS MECHANISM CAN IMPROVE THE UTILIZATION OF IP ADDRESSES AND AVOID THE IP ADDRESS FROM BEING OCCUPIED AFTER THE CLIENT GOES OFFLINE. IF DHCP CLIENTS WANT TO CONTINUE USING THE ADDRESS, THEY NEED TO UPDATE THE LEASE PERIOD OF THE IP ADDRESS (FOR EXAMPLE, EXTENDING THE IP ADDRESS LEASE).

THE DHCP CLIENT UPDATES THE LEASE AS SHOWN IN THE FOLLOWING FIGURE.

*DHCP CLIENT UPDATE LEASE DIAGRAM*

  • WHEN THE LEASE REACHES 50% (T1), THE DHCP CLIENT AUTOMATICALLY SENDS A DHCP REQUEST MESSAGE TO THE DHCP SERVER IN UNICAST TO REQUEST AN UPDATE TO THE IP ADDRESS LEASE. THE LEASE UPDATE IS SUCCESSFUL IF A DHCP ACK PACKET IS RECEIVED IN RESPONSE FROM THE DHCP SERVER (THAT IS, THE LEASE PERIOD STARTS AT 0), AND IF A DHCP NAK PACKET IS RECEIVED, THE DHCP DISCOVER PACKET IS RESENDED REQUESTING A NEW IP ADDRESS.
  • WHEN THE LEASE REACHES 87.5% (T2), IF THE DHCP SERVER STILL DOES NOT RECEIVE A REPLY, THE DHCP CLIENT AUTOMATICALLY SENDS A DHCP REQUEST MESSAGE TO THE DHCP SERVER IN A BROADCAST MANNER REQUESTING AN UPDATE TO THE IP ADDRESS LEASE. THE LEASE UPDATE IS SUCCESSFUL IF A DHCP ACK PACKET IS RECEIVED IN RESPONSE FROM THE DHCP SERVER (THAT IS, THE LEASE PERIOD STARTS AT 0), AND IF A DHCP NAK PACKET IS RECEIVED, THE DHCP DISCOVER PACKET IS RESENDED REQUESTING A NEW IP ADDRESS.
  • IF NO RESPONSE IS RECEIVED FROM THE SERVER AT THE END OF THE LEASE TIME, THE CLIENT STOPS USING THIS IP ADDRESS AND RESENDS THE DHCP DISCOVER MESSAGE REQUEST FOR A NEW IP ADDRESS.

IF THE CLIENT DOES NOT WANT TO USE THE ASSIGNED IP ADDRESS (FOR EXAMPLE, IF THE CLIENT’S NETWORK LOCATION NEEDS TO CHANGE) BEFORE THE LEASE TIME EXPIRES, THE DHCP CLIENT IS TRIGGERED TO SEND A DHCP RELEASE MESSAGE TO THE DHCP SERVER, NOTIFYING THE DHCP SERVER TO RELEASE THE LEASE OF THE IP ADDRESS. THE DHCP SERVER RETAINS THE CONFIGURATION INFORMATION FOR THIS DHCP CLIENT, LISTING THE IP ADDRESS AS THE IP ADDRESS THAT WAS ONCE ASSIGNED SO THAT IT CAN BE SUBSEQUENTLY REASSIGNED TO THAT CLIENT OR TO ANOTHER CLIENT. CLIENTS CAN REQUEST UPDATE CONFIGURATION INFORMATION FROM THE SERVER BY SENDING DHCP INFORM MESSAGES.

AS SHOWN IN THE FOLLOWING FIGURE, WHEN YOU DEPLOY A DHCP RELAY, THE PROCESS FOR UPDATING THE LEASE IS SIMILAR TO THE PROCESS DESCRIBED ABOVE.

3. DHCP USAGE SCENARIOS

DHCP PROVIDES TWO ADDRESS ALLOCATION MECHANISMS, AND NETWORK ADMINISTRATORS CAN CHOOSE DIFFERENT ALLOCATION STRATEGIES FOR DIFFERENT HOSTS BASED ON NETWORK NEEDS.

  • DYNAMIC ALLOCATION MECHANISM: ASSIGNS A HOST A VALID IP ADDRESS VIA DHCP.
  • DHCP uses the concept of lease term, or the validity period of a device IP address. The lease time is indefinite and depends on how long it takes the user to connect to the Internet in a certain place, and this allocation mechanism is suitable for scenarios where the host needs to temporarily access the network or the number of idle addresses is less than the total number of network hosts and the host does not need to connect to the network permanently.
  • STATIC ALLOCATION MECHANISM: THE NETWORK ADMINISTRATOR ASSIGNS A FIXED IP ADDRESS TO THE SPECIFIED HOST VIA DHCP.
  • COMPARED WITH MANUAL STATIC CONFIGURATION OF IP ADDRESSES, THE STATIC ALLOCATION MECHANISM OF DHCP MODE AVOIDS MANUAL CONFIGURATION ERRORS AND FACILITATES UNIFIED MAINTENANCE AND MANAGEMENT BY ADMINISTRATORS.