ITD107 - Week 10 - DHCP.pptx

Introduction to Computer Networks DHCP How to Get IP Address  Devices on a network can get an IP address in either two ways: Static IP assigned manually Dynamically using DHCP  Dynamic Host Configuration Protocol (DHCP) is an application layer protocol used to dynamically assign an...

Introduction to Computer Networks DHCP How to Get IP Address  Devices on a network can get an IP address in either two ways: Static IP assigned manually Dynamically using DHCP  Dynamic Host Configuration Protocol (DHCP) is an application layer protocol used to dynamically assign an IP address to any device, or node, on a network so it can communicate using IP DHCP Server  DHCP servers assign IP addresses to hosts  This protocol gives us a much easier way to administrate by automatically providing IP information to hosts  It works well in any network environment, from tiny to huge, and allows all types of hardware to be employed as a DHCP server, including routers.  In small organizations or home network, routers can be configured to provide DHCP services without the need for dedicated server 4 DHCP Server The DHCPv4 server dynamically assigns, or leases, an IPv4 address from a pool of addresses for a limited period of time chosen by the server, or until the client no longer needs the address. Clients lease the information from the server for a defined period. Administrators configure DHCPv4 servers to set the leases to time out at different intervals. The lease is typically anywhere from 24 hours to a week or more. When the lease expires, the client must ask for another address, although the client is typically reassigned the same address. 5 DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover Arriving client Broadcast: is there a DHCP server out there? Unicast: I’m a DHCP DHCP offer server! Here’s an IP address you can use DHCP request Broadcast: OK. I would like to use this IP address! DHCP ACK Unicast: OK. You’ve got that IP address! 6 DHCP client-server scenario 1. The DHCP client broadcasts a DHCP Discover message looking for a DHCP server (Port 67). 2. The DHCP server that received the DHCP Discover message sends a unicast DHCP Offer message back to the host 3. The client then broadcasts to the server a DHCP Request message asking for the offered IP address and possibly other information. 4. The server finalizes the exchange with a unicast DHCP Acknowledgment message. 7 DHCP: example DHCP DHCP  Connecting laptop will use DHCP DHCP UDP IP DHCP to get IP address, DHCP Eth address of first-hop router, Phy address of DNS server. DHCP  DHCP REQUEST message DHCP DHCP 168.1.1.1 encapsulated in UDP, DHCP UDP encapsulated in IP, DHCP IP Eth encapsulated in Ethernet DHCP router with DHCP Phy server built into  Ethernet frame broadcast (dest: router FFFFFFFFFFFF) on LAN, received at router running DHCP server  Ethernet de-encapsulate to IP de-encapsulate UDP de- encapsulate to DHCP 8 DHCP: example DHCP DHCP  DCP server formulates DHCP DHCP UDP ACK containing client’s IP DHCP IP address, IP address of first-hop Eth router for client, name & IP DHCP Phy address of DNS server  Encapsulated DHCP server DHCP DHCP reply forwarded to client, de- DHCP DHCP UDP IP encapsulate up to DHCP at DHCP Eth router with DHCP client Phy server built into DHCP router  Client now knows its IP address, name and IP address of DNS server, IP address of its first-hop router 9 DHCP: more than IP addresses  DHCP can return more than just allocated IP address on subnet: Address of first-hop router for client Name and IP address of DNS sever Network mask (indicating network versus host portion of address) 10 Renew a Lease  Prior to lease expiration, the client begins a two-step process to renew the lease with the DHCPv4 server, as shown in the figure:  DHCP Request (DHCPREQUEST) The client sends a DHCPREQUEST message directly to the DHCPv4 server that originally offered the IPv4 address. If a DHCPACK is not received within a specified amount of time, the client broadcasts another DHCPREQUEST so that one of the other DHCPv4 servers can extend the lease  DHCP Acknowledgment (DHCPACK) On receiving the DHCPREQUEST message, the server verifies the lease information by returning a DHCPACK. 11 DHCP Relay Agent  In large network settings, network clients are not on the same subnet as servers providing services like DHCP server.  Routers don’t propagate broadcast messages.  Any network client attempting to acquire an IPv4 address from a DHCP server on other subnets using a broadcast message will fail  DHCP relay agent is then used in this case  DHCP relay agent relays DHCP messages between DHCP clients and DHCP servers on different IP networks 12 DHCP Relay Agent  Router A is configured with the IP helper address command on interface E0 of the router.  Whenever interface E0 receives a broadcast request, Router A will forward those requests as a unicast  you can configure Router A to forward these requests and even use multiple DHCP servers for redundancy, if needed 13