Configuring a DHCP Server DHCP Operation

Transcription

1 CCNA4 Chapter 7 * Configuring a DHCP Server The steps to configure a router as a DHCP server: Step 1. Define a range of addresses that DHCP is not to allocate. These are usually static addresses reserved for the router interface, switch, servers, and local network printers. Router(config)#ip dhcp excluded-address Step 2. Create the DHCP pool Router(config)#ip dhcp pool command. Step 3. Configure the specifics of the pool. You must configure the available addresses. You should also define the default gateway for the clients. Optional DHCP pool commands: DNS server DHCP lease: default setting is one day, * DHCP Operation The DHCP statistics are also reflecting the exchange of DHCP messages. DHCPDISCOVER (Broadcast) The DHCPDISCOVER message finds DHCP servers on the network. DHCPOFFER (Unicast) When the DHCP server receives a DHCDISCOVER message, it finds an available IP address to lease and transmits a binding offer with a DHCPOFFER message. DHCPREQUEST (Broadcast) When the client receives the DHCPOFFER from the server, it sends back a DHCPREQUEST message. servers about the accepted offer. DHCPACK (Unicast) When the client receives the DHCPACK message, it knows that the IP address is valid and starts using it as its own.

2 * NAT It saves IP addresses by allowing networks to use private IP addresses. NAT adds a degree of privacy and security to a network because it hides internal IP addresses from outside networks. * Differences Between NAT and NAT Overload (PAT) NAT: It generally only translates IP addresses on a 1:1 correspondence between publicly exposed IP addresses and privately held IP addresses. NAT overload: It modifies both the private IP address and port number of the sender. NAT overload chooses the port numbers seen by hosts on the public network. This is what most home routers do. * Dynamic Mapping and Static NAT Mapping Dynamic: Dynamic NAT uses a pool of public addresses and assigns them on a first-come, first-served basis. When a host with a private IP address requests access to the Internet, dynamic NAT chooses an IP address from the pool that is not already in use by another host. Static: Static NAT uses a one-to-one mapping of local and global addresses, and these mappings remain constant. Static NAT is particularly useful for servers or hosts that must have a consistent address that is accessible from the Internet. * NAT terminology Inside local address Usually an RFC 1918 address. The "inside" of a NAT configuration is not synonymous with private addresses as defined by RFC Inside global address - Valid public address that the inside host is given when it exits the NAT router. Outside global address - Reachable IP address assigned to a host on the Internet. Outside local address - The local IP address assigned to a host on the outside network. Refer to the figure, which one is Inside local, or global local?

3 * Configuring Static NAT (See the figure above) 1. You need to define the addresses to translate 2. Configure NAT on the appropriate interfaces. * Configuring Dynamic NAT Dynamic NAT configuration differs from static NAT, Rather than creating a static map to a single IP address, a pool of inside global addresses is used (make sure you know how the pool is configured. You need to know how to read it and interpreted it). To configure dynamic NAT, you need an ACL to permit only those addresses that are to be translated. * Configuring NAT Overload for a Single Public IP Address The primary difference between this configuration and the configuration for dynamic and one-to-one NAT is that the overload keyword is used. * Verifying NAT and NAT Overload One of the most useful commands when verifying NAT operation is the show ip nat translations. Before using the show commands to verify NAT, you must clear any dynamic translation entries that might still be present using clear ip nat translation command. By default, translation entries time out after 24 hours * Port Forwarding

4 NAT does not allow requests initiated from the outside. Port forwarding allows you to identify specific ports that can be forwarded to inside hosts. Port forwarding (sometimes referred to as tunneling) It is the act of forwarding a network port from one network node to another. * Time Improved features for IPv6 Better routing efficiency No broadcasts and no threat of broadcast storms * IPv6 Transition Strategies Dual Stacking Dual stacking is an integration method in which a node has connectivity to both an IPv4 and IPv6 network. Tunneling Dynamic 6to4 tunneling - Automatically establishes the connection of IPv6 islands through an IPv4 network NAT-Protocol Translation (NAT-PT) This translation allows direct communication between hosts that use different versions of the IP protocol. * Configured IPv6 There are two basic steps to activate IPv6 on a router. First, you must activate IPv6 traffic-forwarding on the router, By default, IPv6 traffic-forwarding is disabled on a Cisco router. To activate it between interfaces, you must configure the global command ipv6 unicast-routing. Second, you must configure each interface that requires IPv6. The ipv6 address command can configure a global IPv6 address. The link-local address is automatically configured when an address is assigned to the interface. You must specify the entire 128-bit IPv6 address or specify to use the 64-bit prefix by using the eui-64 option. * IPv6 Address Management IPv6 addresses use interface identifiers to identify interfaces on a link.

5 Interface identifiers are always 64 bits and can be dynamically derived from a Layer 2 address (MAC). The EUI-64 standard explains how to stretch IEEE 802 MAC addresses from 48 to 64 bits by inserting the 16-bit 0xFFFE in the middle at the 24th bit of the MAC address to create a 64-bit, unique interface identifier. You can completely specify the IPv6 address or compute the host identifier (rightmost 64 bits) from the EUI-64 identifier of the interface. * IPv6 Addressing IPv6 Global Unicast Address Global unicast addresses typically consists of a 48-bit global routing prefix and a 16-bit subnet ID. (for example: 2001:db8:1234::/48 ) Individual organizations can use a 16-bit subnet field to create their own local addressing hierarchy. This allows an organization to use up to 65,535 subnets.