IP Services
IP Services
IP Services covers the supporting protocols that make a network usable and observable: NAT, DHCP, NTP, DNS, SNMP, syslog, QoS marking, and file or remote-access services.
Core rules
- NAT terms: inside local (private host), inside global (public address after translation), outside global (remote host). PAT/overload multiplexes many inside hosts on one global address using source ports.
- Translation direction matters: inside-to-outside rewrites the source, outside-to-inside rewrites the destination. Interfaces need ip nat inside or ip nat outside.
- DHCP DORA: Discover (broadcast), Offer, Request, Acknowledge. To reach a server on another subnet, the client-facing interface needs ip helper-address.
- Syslog severities: 0 emergency, 1 alert, 2 critical, 3 error, 4 warning, 5 notice, 6 informational, 7 debug. Lower number is more severe.
- NTP stratum 1 attaches directly to a reference clock; stratum 16 means unsynchronized. SNMPv3 adds authentication and encryption; v1/v2c send community strings in clear text.
Worked example
Translate 10.1.1.10 to 203.0.113.5 for all outbound traffic: access-list 1 permit 10.1.1.0 0.0.0.255 ip nat inside source list 1 interface GigabitEthernet0/1 overload Overload rewrites source IP and source port. show ip nat translations displays inside local, inside global, and outside addresses so the mapping can be verified.
Traps
Forgetting the NAT ACL, so nothing matches and nothing translates; assuming DHCP broadcasts cross a router by default; reversing syslog severity numbers; and treating NTP stratum as a quality score rather than distance from the reference clock.