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Dynamic Host Configuration Protocol DHCP


DHCP is an extension of Boot Protocol (BOOTP). Allows diskless clients to configure TCP/IP automatically. Centralizes and manages the allocation of TCP/IP. ... – PowerPoint PPT presentation

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Title: Dynamic Host Configuration Protocol DHCP

Dynamic Host Configuration Protocol (DHCP)
  • DHCP is an extension of Boot Protocol (BOOTP).
  • Allows diskless clients to configure TCP/IP
  • Centralizes and manages the allocation of TCP/IP.
  • Automatically assigns addresses ( other IP
    config settings)

IP config is set manually
DHCP clients get IP config settings from DHCP
Problems with configuring TCP/IP manually
  • Administrative overhead
  • Need to keep track of all IP assignments
  • Need to manually enter IP settings
  • Bad or duplicate IP addresses
  • Mistakes will happen!
  • Moving between subnets
  • Need to manually change IP settings

ConfiguringTCP/IP with DHCP
  • Benefits of DHCP
  • Centralized management
  • Automatic supply of address information to
  • Easier to troubleshoot
  • Example settings that DHCP can provide
  • IP address for network adapter
  • Subnet masks
  • Default gateways
  • Additional parameters DNS, WINS, others

Four Phases of DHCP Client Configuration
  • IP lease discover
  • client initializes limited version of TCP/IP and
    broadcasts a request for the location of a DHCP
    server and IP addressing information
  • IP lease offer
  • all DHCP servers that have valid IP addressing
    info. for the client send an offer to the client
  • IP lease request
  • the client selects the IP addressing info. From
    the 1st offer broadcasts a message requesting a
    lease on the IP address ( other IP settings) in
    the offer
  • IP lease acknowledgement
  • The DHCP server that made the offer responds and
    an ACK the client can then bind the protocol
    start using the IP settings in the lease. All
    other DHCP servers withdraw their offers.

IP Lease Discover and Offer
Sending a DHCPOFFER Message
At this point, client has no IP address, knows
nothing of what network its on (thus the
broadcast On subnet
Routers can use the Offered IP Address to route
the DHCPOFFER to the correct subnet
When No DHCP Servers Online
  • Client waits 1 second.
  • Client rebroadcasts at 9, 13, 16 seconds and then
    at random intervals.
  • Client retries every 5 minutes.

Automatic Private IP Addressing (APIPA)
  • APIPA occurs when client attempts request, and no
    DHCP server responds
  • Autoconfiguration of a Class B address.
  •, with subnet mask
  • APIPA generates a valid IP address on this
  • Clients tests for conflicts (i.e. IP address
    already in use)
  • If theres a conflict, repeat last step until no
  • Client rechecks for a DHCP server every 5
  • Useful only when you have a single, non-routed
  • Purpose allows small LANs to be put together by
    novices (NetBEUI served a similar purpose in
    Windows 3.11, and NT prior to NT 4)

IP Lease Request
  • Client broadcasts DHCPREQUEST message.
  • Broadcast can be forwarded to all DHCP servers
  • Other DHCP servers retract offers.

  • DHCP server that originally made the offer
    normally sends DHCPACK to DHCP client that sent
  • Client becomes a bound DHCP client.
  • DHCPNACK (negative acknowledgement) can occur
    when a client is trying to lease its previous IP
  • The IP address is no longer available.
  • The client has been moved to a different subnet.
  • If a DHCPNACK is received, the client returns to
    the process of requesting an IP lease.

Before Installing DHCP
  • Hardware and storage requirements
  • Will routers support DHCP forwarding?
  • How many DHCP servers needed?
  • On which subnets will they located?
  • Which computers to configure via DHCP
  • Most client computers
  • Which computers to configure manually
  • Most servers use static IP addresses
  • DHCP options and values to be predefined

Options that Determine Configuration
  • IP address and Mask are always part of the IP
  • Other options include
  • Default gateway
  • Domain Name System (DNS) server
  • Windows Internet Name Service (WINS) server
  • NetBIOS over TCP/IP name resolution
  • In what order is it done, which components
  • e.g. broadcast then WINS, or WINS then broadcast
  • Is broadcast name resolution allowed.

Ipconfig options
  • /? Display help message
  • /all Display full configuration
  • /release Release the IP address for the
    specified adapter.
  • /renew Renew the IP address for the
    specified adapter.
  • /flushdns Purges the DNS Resolver cache.
  • /registerdns Refreshes all DHCP leases and
    re-registers DNS names
  • /displaydns Display the contents of the DNS
    Resolver Cache.

DHCP Relay Agent
  • DHCP broadcasts may not, by default, be forwarded
    to other subnets by routers.
  • Many routers can be configured to pass DHCP/BOOTP
    messages to other segments
  • referred to as BOOTP Relay
  • Allows for centralization of DHCP services in one
    place or on selected subnets
  • Windows NT/2000/2003 can be configured as a DHCP
    Relay Agent
  • Forwards DHCP messages between clients and
    servers on subnets, useful when routers wont do
    BOOTP Relay
  • If neither of the above, need to set up separate
    DHCP servers for each subnet

How DHCP Servers Provide Optional Data
  • Default gateways that connect network segments
  • IP addresses for DNS servers
  • IP addresses for WINS servers
  • NetBIOS over TCP/IP settings (Node Type)

Installing DHCP
  • Install Microsoft DHCP Server service.
  • Authorize the DHCP server.
  • Configure global settings
  • Global (Server) settings those that will be
    applied by default to all scopes.
  • Simplifies scope administration if you want
    scopes to have the same settings. Common
  • DNS WINS servers
  • NetBIOS over TCP/IP settings (Node Type)
  • Configure one or more scopes

Authorizing a DHCP Server
  • Windows 2003 servers are verified.
  • DHCP servers are authorized.
  • First server in Active Directory
  • Installed as domain controller or member
    servernot stand-alone
  • Authorization process depends on server role.
  • Domain controller
  • Member server
  • Stand-alone server

Configuring a scope
  • A scope corresponds to an IP sub-network
  • Defines a pool (a range) of addresses to be
    allocated to hosts on the subnet
  • Define range to exclude addresses that are
    statically assigned, two approaches
  • Define range endpoints to exclude others for
    static assignment, e.g. On use through leaves to for static assignment
  • Exclude specific IP address ranges from the
    range, e.g. for range through, exclude through

Configuring a scope
  • Define other IP settings for the scope.
  • If global settings are used, scope settings
  • If no global settings are defined, scope settings
    are the only ones that are used.
  • Scope-level settings typically used when settings
    for the subnet differ from other subnets.
  • Default gateway is a good example.

DHCP Scope
  • (At least) One scope for every DHCP server.
  • Exclude static IP addresses from scope.
  • Multiple scopes will centralize administration.
  • One scope to a subnet (on the DHCP server)
  • DHCP servers do not share scope information.
  • IP addresses must be unique to scope (cant exist
    in other scopes)
  • Determine starting and ending addresses.

(No Transcript)
Scope and IP Address Ranges for Server A and
Server B
Registering for DNS Updates
  • Windows 2000/2003/XP can register with a DNS
  • Supports DNS Dynamic update protocol for
    automatic record updating.
  • DHCP acts as registration proxy.
  • DHCP and static DNS are not compatible.
  • DHCP servers provide default support for legacy
    DHCP clients in DNS zones.
  • The next two slide
  • DHCP/DNS update interaction for Windows
    2000/2003/XP hosts
  • DHCP/DNS update interaction for pre-Windows 2000
    hosts (Windows NT 4 and below)

A DHCP Client Interacting with the DNS Dynamic
Update Protocol
DHCP/DNS Interaction with Older Windows Clients
Number of DHCP Servers
  • Size of network
  • Number of DHCP-enabled clients
  • Transmission speed between network segments
  • Speed of network links
  • IP address class of the network
  • Isolated or multiple DHCP servers

Troubleshooting - Invalid IP Address
  • Client does not have IP address, or client has IP
    address of 169.254.x.x.
  • Results from client not being able to contact a
    DHCP server.
  • Windows could not find a DHCP server and provided
    APIPA IP address.
  • Consider disabling APIPA if it isnt used
  • Determine whether network hardware failure or
    DHCP server is unavailable.
  • Verify that client has valid functioning network
  • Check hardware.

Troubleshooting Missing Configuration Details
  • Client is missing DHCP options.
  • e.g default gateway, DNS Server, etc.
  • Verify that options distribution is configured
    properly on the DHCP servers handling IP
    addresses for the scopes experiencing problems
  • Check DHCP option settings.

Preventing DHCP Problems
  • Use the 80/20 design rule for balancing scope
    distribution of addresses where multiple DHCP
    servers are used to service the same scope.
  • When a scope is handled by gt 1 DHCP server, make
    sure address ranges DO NOT OVERLAP!!!!
  • Create reservations on all DHCP servers that can
    potentially service the reserved client.
  • DHCP is disk-intensive purchase hardware with
    optimal disk performance characteristics.
  • Keep audit logging enabled for use in

Thats all folks !
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