1. Allocating IP Addressing by Using Dynamic Host Configuration Protocol

2. Overview The Role of DHCP in the Network Infrastructure Adding and Authorizing the DHCP Server Service Configuring a DHCP Scope Configuring DHCP Reservations and Options Configuring a DHCP Relay Agent

3. 3 Dynamic Assignment of IP addresses Dynamic assignment of IP addresses is desirable for several reasons:  IP addresses are assigned on-demand  Avoid manual IP configuration  Support mobility of laptops

4. 4 Solutions for dynamic assignment of IP addresses Reverse Address Resolution Protocol (RARP)  Works similar to ARP  Broadcast a request for the IP address associated with a given MAC address  RARP server responds with an IP address  Only assigns IP address (not the default router and subnetmask)

5. 5 BOOTP BOOTstrap Protocol (BOOTP) From 1985 Host can configure its IP parameters at boot time. 3 services. IP address assignment. Detection of the IP address for a serving machine. The name of a file to be loaded and executed by the client machine (boot file name)  Not only assign IP address, but also default router, network mask, etc.  Sent as UDP messages (UDP Port 67 (server) and 68 (host))  Use limited broadcast address (255.255.255.255): These addresses are never forwarded

6. 6 DHCP Dynamic Host Configuration Protocol (DHCP)  From 1993  An extension of BOOTP, very similar to DHCP  Same port numbers as BOOTP  Extensions: Supports temporary allocation (“leases”) of IP addresses DHCP client can acquire all IP configuration parameters needed to operate  DHCP is the preferred mechanism for dynamic assignment of IP addresses  DHCP can interoperate with BOOTP clients.

7. 7 BOOTP Interaction BOOTP can be used for downloading memory image for diskless workstations Assignment of IP addresses to hosts is static (a) (b) (c)

8. 8 DHCP Interaction (simplified)

9. 9 BOOTP/DHCP Message Format (There are >100 different options)

10. 10 BOOTP/DHCP OpCode: 1 (Request), 2(Reply) Note: DHCP message type is sent in an option Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to boot Client IP address, Your IP address, server IP address, Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank

11. Lesson: Adding and Authorizing the DHCP Server Service Why Use DHCP? What Is Automatic Private IP Addressing? How DHCP Allocates IP Addresses How the DHCP Lease Generation Process Works How the DHCP Lease Renewal Process Works How a DHCP Server Service Is Authorized Practice: Adding and Authorizing a DHCP Server Service

12. Why Use DHCP? DHCP reduces the complexity and amount of administrative work by using automatic TCP/IP configuration Manual TCP/IP Configuration IP addresses are entered manually IP address could be entered incorrectly Communication and network issues can result Frequent computer moves increase administrative effort IP addresses are entered manually IP address could be entered incorrectly Communication and network issues can result Frequent computer moves increase administrative effort Automatic TCP/IP Configuration IP addresses are supplied automatically Correct configuration information is ensured Client configuration is updated automatically A common source of network problems is eliminated IP addresses are supplied automatically Correct configuration information is ensured Client configuration is updated automatically A common source of network problems is eliminated

13. What Is Automatic Private IP Addressing? APIPA automatically self-configures addresses when there is no DHCP server available Advantages Serves as a DHCP server failover mechanism for small networks Automatically assigns an IP address in a specific range Serves as a DHCP server failover mechanism for small networks Automatically assigns an IP address in a specific range Disadvantages Forces assignment of addresses typically not used Conceals possible connectivity problems Does not work outside 169.254.x.x subnet Is not routable Forces assignment of addresses typically not used Conceals possible connectivity problems Does not work outside 169.254.x.x subnet Is not routable

14. How DHCP Allocates IP Addresses DHCP Server DHCP Database IP Address1: Leased to DHCP Client1 IP Address2: Leased to DHCP Client2 IP Address3: Available to be leased DHCP Client2: IP configuration from DHCP server Non-DHCP Client: Static IP configuration DHCP Client1: IP configuration from DHCP server Lease Renewal Lease Generation

15. How the DHCP Lease Generation Process Works DHCP client broadcasts a DHCPDISCOVER packet 1 1 DHCP servers broadcast a DHCPOFFER packet 2 2 DHCP client broadcasts a DHCPREQUEST packet 3 3 DHCP Server1 broadcasts a DHCPACK packet 4 4 DHCP Client DHCP Server1 DHCP Server2 DHCP client broadcasts a DHCPDISCOVER packet 1 1 DHCP servers broadcast a DHCPOFFER packet 2 2 DHCP client broadcasts a DHCPREQUEST packet 3 3 DHCP Server1 broadcasts a DHCPACK packet 4 4 DHCP Client DHCP Server1 DHCP Server2

16. How the DHCP Lease Renewal Process Works DHCP Client DHCP Server1 DHCP Server2 DHCP Client sends a DHCPREQUEST packet 1 1 DHCP Server1 sends a DHCPACK packet 2 2 50% of lease duration has expired 87.5% of lease duration has expired 100% of lease duration has expired If the client fails to renew its lease, after 50% of the lease duration has expired, then the DHCP lease renewal process will begin again after 87.5% of the lease duration has expired If the client fails to renew it’s lease, after 87.5% of the lease has expired, then the DHCP lease generation process starts over again with a DHCP client broadcasting a DHCPDISCOVER DHCP Client DHCP Server1 DHCP Server2 DHCP client sends a DHCPREQUEST packet 1 1 DHCP Server1 sends a DHCPACK packet 2 2 50% of lease duration has expired

17. How a DHCP Server Service Is Authorized Domain Controller Domain Controller Active Directory Active Directory DHCP Client DHCP Server1 checks with the domain controller to obtain a list of authorized DHCP servers Unauthorized Does not service DHCP requests Authorized Services DHCP requests DHCP Server1 DHCP Server2 If DHCP Server1 finds its IP address on the list, the service starts and supports DHCP clients DHCP Server2 checks with the domain controller to obtain a list of authorized DHCP servers DHCP Server2 checks with the domain controller to obtain a list of authorized DHCP servers If DHCP Server2 does not find its IP address on the list, the service does not start and support DHCP clients DHCP client receives IP address from authorized DHCP Server1 DHCP client receives IP address from authorized DHCP Server1 DHCP authorization is the process of registering the DHCP Server service in the Active Directory domain to support DHCP clients

18. What Are DHCP Scopes? A scope is a range of IP addresses that are available to be leased Scope Properties Scope name Exclusion range Lease duration Network IP address range Network ID Subnet mask LAN ALAN B DHCP Server Scope B Scope A

19. What Is Network Monitor? Network Monitor: Captures a sample of network traffic Uses filters to select specific packets Decodes the packets Compiles network statistics 2 2 4 4 3 3 1 1

20. What Is a DHCP Reservation? A reservation is a specific IP address, within a scope, that is permanently reserved for lease to a specific DHCP client Subnet A Subnet B Workstation 1 DHCP Server Workstation 2 File and Print Server IP Address1: Leased to Workstation 1 IP Address2: Leased to Workstation 2 IP Address3: Reserved for File and Print Server IP Address1: Leased to Workstation 1 IP Address2: Leased to Workstation 2 IP Address3: Reserved for File and Print Server

21. What Are DHCP Options? DHCP options are configuration parameters that a DHCP server assigns to clients DHCP Client DHCP Server DHCP Client IP Configuration Data Client’s IP address Client’s subnet mask DHCP options Client’s IP address Client’s subnet mask DHCP options

22. How DHCP-Server, Scope, and Reserved-Client Options Are Applied DHCP option applied at the server level DHCP option applied at the server level DHCP option applied at the scope level DHCP option applied at the scope level DHCP option applied at the reserved-client level DHCP option applied at the reserved-client level Scope A Scope B Windows XP DHCP Server Windows 98 Windows XP Router File and Print Server Scope A Scope B Windows XP DHCP Server Windows 98 Windows XP Router File and Print Server

23. How DHCP Class-Level Options Are Applied DHCP option applied at the class level DHCP option applied at the class level Scope A Scope B DHCP Server Windows 98 Windows XP Router File and Print Server

24. Lesson: Configuring a DHCP Relay Agent What Is a DHCP Relay Agent? How a DHCP Relay Agent Works How a DHCP Relay Agent Uses Hop Count How a DHCP Relay Agent Uses Boot Threshold Practice: Configuring a DHCP Relay Agent

25. What Is a DHCP Relay Agent? A DHCP relay agent is a computer or router that listens for DHCP/BOOTP broadcasts from DHCP clients and then relays those messages DHCP Server Client DHCP Relay Agent Client Routers (Non–RFC 1542 Compliant) Routers (Non–RFC 1542 Compliant) Unicast Broadcast Subnet ASubnet B Broadcast

26. How a DHCP Relay Agent Works Router Non-RFC 1542 Compliant Router Non-RFC 1542 Compliant Client1 DHCP Relay Agent Client2 DHCP Server Client3 Router (Non–RFC 1542 Compliant) Router (Non–RFC 1542 Compliant) Client1 DHCP Relay Agent Client2 DHCP Server Client3 Client1 broadcasts a DHCPDISCOVER packet 1 1 Relay agent forwards the DHCPDISCOVER message to the DHCP server 2 2 Server sends a DHCPOFFER message to the DHCP relay agent 3 3 Relay agent broadcasts the DHCPOFFER packet 4 4 Client1 broadcasts a DHCPREQUEST packet 5 5 Relay agent forwards the DHCPREQUEST message to the DHCP server 6 6 Server sends a DHCPACK message to the DHCP relay agent 7 7 Relay agent broadcasts the DHCPACK packet 8 8

27. How a DHCP Relay Agent Uses Hop Count The hop count threshold is the number of routers through which the packet can be transmitted before it is discarded DHCP Relay Agent 2 DHCP Server Hop Count = 2 DHCP Relay Agent 1

28. How a DHCP Relay Agent Uses Boot Threshold DHCP Server 2 DHCP Server 3 DHCP Relay Agent Boot Threshold = 10 seconds Local DHCP Server The boot threshold is the time the DHCP relay agent will wait for a DHCP server response before forwarding the request