4 IP Services and Address Management
Learn how IP addressing, DHCP, IPv6 autoconfiguration, DNS, NAT, and NTP support network connectivity and how to verify them during troubleshooting.
IP connectivity and troubleshooting
A host needs a valid IP address and prefix, a functioning interface, and a path to its destination. IPv4 addresses are bits, while IPv6 addresses are bits.
A host sends traffic directly to destinations on its local subnet. For a remote subnet, it sends the packet to its , which must have a route onward. A correct address alone does not guarantee connectivity: link status, VLAN membership, address and prefix length, gateway reachability, routes, and filtering can also affect communication.
A troubleshooting sequence
Verify the local interface and address.
Ping the local gateway.
Test a remote IP address.
Test a name.
If a remote IP address responds but its name does not, investigate name resolution rather than routing. On Cisco devices, useful checks include show ip interface brief, show ipv6 interface brief, show ip route, and show ipv6 route.
Address assignment and IPv4
Addresses can be configured manually or assigned dynamically. Static addressing is useful when a device needs a predictable address, such as a router interface or server; ensure that the address does not conflict with a pool. reduces manual configuration for clients and can supply a subnet mask, , servers, and lease duration.
IPv4 operation
The common exchange is : the client broadcasts Discover, a server replies with Offer, the client sends Request, and the server returns Acknowledgment. A lease is temporary, and clients renew it periodically. uses UDP port 67 for the server and 68 for the client.
Initial client broadcasts do not cross routers. If the server is on another subnet, configure a relay on the client-facing Layer 3 interface; Cisco IOS commonly uses ip helper-address for this purpose.
Example server and relay configuration
This server configuration excludes a range that can reserve addresses such as the gateway and statically addressed infrastructure:
For a remote server, configure the relay on the LAN gateway interface:
Check bindings, pool information, and configuration with show ip binding, show ip pool, and show running-config.
IPv6 address assignment
IPv6 hosts commonly use : a router advertises a prefix, and a host forms an address and checks for duplicates. Hosts learn default-router information from Router Advertisements (RAs), which can also advertise prefixes and support address autoconfiguration. IPv6 Neighbor Discovery (NDP), carried in ICMPv6, supports functions such as discovering routers and resolving neighbors.
can operate in two ways. Stateful assigns and tracks addresses. Stateless supplies additional settings, such as information, while supplies the address. does not replace RAs for default-router discovery.
A network can use alone, with stateless for extra options, or stateful for addresses and other options. RA flags guide host behavior, though exact client behavior can vary by operating system. On supported Cisco IOS platforms, ipv6 address autoconfig enables client-side autoconfiguration on an interface.
name resolution
The Domain Name System () maps names to records that clients can use to locate services. When an application needs an address for a name, a resolver queries . is hierarchical and distributed: a client typically asks a configured recursive resolver, which may use cached information or query other servers.
Common record types include:
A: IPv4 address.
AAAA: IPv6 address.
CNAME: alias.
MX: mail exchanger.
PTR: reverse lookup.
commonly tells clients which resolver to use, but and are separate services. A successful lease does not prove that name resolution works. If an IP address is reachable but a hostname is not, check the configured resolver, reachability, and whether the needed record exists. normally uses UDP port 53 for queries and can use TCP port 53 when needed.
and address translation
changes IP address information as packets pass through a router. For inside-source , Cisco terminology distinguishes the inside local address, used for the inside host internally, from the inside global address, which represents that host externally.
Static creates a fixed one-to-one mapping, useful when an internal service must be reachable at a consistent external address.
Dynamic assigns addresses from a configured public-address pool as needed.
, also called overload, lets many inside hosts share one public IPv4 address by distinguishing sessions with transport-layer ports.
A basic configuration uses an access list to select inside addresses, marks interfaces as inside or outside, and translates eligible sources to the outside interface address:
A matching interface configuration and a working route to the outside are required. Check translations and counters with show ip translations and show ip statistics. conserves public IPv4 addresses, but it is not by itself a firewall or a substitute for security policy.
and network time
Network Time Protocol () synchronizes device clocks across a network. Accurate time makes logs, event correlation, and troubleshooting more reliable. clients synchronize with servers, and servers can in turn synchronize with upstream sources.
indicates distance from a reference clock in the synchronization hierarchy; it is not a direct measure of time accuracy. uses UDP port 123 and supports IPv4 and IPv6.
A Cisco IOS device can be configured as a client of an approved time server:
Check synchronization and server associations with show status and show associations. A configured server does not guarantee synchronization: verify IP reachability, routing, filtering of UDP port 123, and the device’s reported status.