IP Services
Course Description
IP services are the protocols that keep an IP network running behind the scenes. Hosts need ARP to find MAC addresses, DHCP to get an IP address, DNS to resolve hostnames, and a default gateway that doesn’t fail. Your private addresses need NAT or PAT to reach the Internet, and your devices need the correct time. In this course you’ll learn how these services work and how to configure, verify, and troubleshoot them on Cisco IOS. We also cover IP SLA to measure network performance, BFD for fast failure detection, and WCCP to redirect web traffic to a proxy server.
Course Highlights
In this course you will learn:
- How ARP resolves MAC addresses, and how RARP and BOOTP led to DHCP.
- How DHCP works, and how to configure a Cisco IOS DHCP server, relay agent, client, and static bindings.
- How to configure static NAT, dynamic NAT, PAT, and port forwarding.
- Advanced NAT topics like policy NAT, the NAT Virtual Interface, NAT on a stick, and NAT ALG.
- How HSRP, VRRP, and GLBP give your hosts a redundant default gateway.
- How to tune HSRP with timers, preemption, object tracking, authentication, and BFD.
- How to synchronize clocks with NTP, and when you need PTP instead.
- How to measure network performance with IP SLA and detect failures fast with BFD.
- How to troubleshoot DHCP, NAT and PAT, HSRP, VRRP, and NTP.
Presented to you by instructor Rene Molenaar, CCIE #41726
Course at a glance
- 47 lessons in 8 units.
- Format: self-paced lessons with Cisco IOS configurations, packet captures, and video.
- Last updated: October 2026.
Requirements
- Basic knowledge of IP addressing and subnetting.
- Basic routing knowledge and Cisco IOS configuration skills.
Course Schedule
What are IP services?
IP services are the supporting protocols that make an IP network usable. They don’t route packets themselves, but without them, nothing works. ARP finds the MAC address that belongs to an IP address, DHCP assigns IP addresses, a default gateway, and DNS servers to hosts, and DNS translates hostnames to IP addresses. NAT and PAT translate private IP addresses so that your hosts can reach the Internet with one or a few public IP addresses.
Some IP services are about availability. Hosts can only configure one default gateway, so first hop redundancy protocols like HSRP, VRRP, and GLBP let two or more routers share a virtual gateway address; when one router fails, the other takes over. NTP keeps the clocks of your devices in sync, which you need for log timestamps and certificates. IP SLA measures performance and reachability, and BFD detects link failures much faster than routing protocol hello packets.
Who needs IP services? Every network engineer. You configure DHCP, NAT, and gateway redundancy on almost every network, from a small office to a large campus. Many day-to-day problems turn out to be a DHCP, NAT, or HSRP issue.
IP Services Learning Roadmap
This roadmap follows the course units. Each topic has a short explanation and links to the lessons where we cover it in detail.
Address Resolution
When a host wants to send an IP packet on the LAN, it needs the destination MAC address. ARP finds it by sending a broadcast with the IP address it wants to reach. RARP does the opposite: old diskless workstations used it to find their own IP address. We don’t use RARP anymore; it was replaced by BOOTP and later by DHCP.
DHCP
DHCP, the successor of BOOTP, assigns IP addresses, a default gateway, and DNS servers to hosts with four messages: discover, offer, request, and acknowledgment. A Cisco IOS router can be a DHCP server, a DHCP client, or a relay agent that forwards DHCP broadcasts to a server in another subnet. You also learn how to give hosts a fixed address with static bindings, how Cisco IOS uses its own client identifier, what happens when a Windows host gets no answer and falls back to APIPA, and how to troubleshoot DHCP.
NAT and PAT
NAT translates IP addresses, and PAT also translates port numbers so that many hosts can share a single public IP address. We start with static NAT, dynamic NAT, and PAT, and then look at the difference between ip nat inside source and ip nat outside source. Policy NAT uses route-maps for more specific rules, and the NAT Virtual Interface replaces the inside and outside domains. Some applications like FTP and SIP carry IP addresses in their payload, which is why we need NAT ALG. When a translation doesn’t work, you’ll learn how to troubleshoot NAT and PAT.
Gateway Redundancy with HSRP
A host can only use one default gateway, so when that router fails, the host can’t leave its own subnet unless you have gateway redundancy. HSRP is a Cisco proprietary protocol where two or more routers share a virtual IP and MAC address; one router is active, another is standby, and the HSRP state machine shows how routers get there. The router with the highest priority becomes active, but only takes over from a running active router when you enable preemption. Object tracking lowers the priority when an interface or IP SLA probe fails, and BFD detects a failed peer much faster than the default 10 second hold timer. We also configure HSRP for IPv6, and when the active router isn’t the one you expect, you’ll learn how to troubleshoot HSRP.
VRRP, GLBP and Load Sharing
VRRP is the IETF standard alternative to HSRP. It works in a similar way, but uses master and backup instead of active and standby, and preemption is enabled by default. With HSRP and VRRP, only one router per group forwards traffic, so for load sharing you configure multiple HSRP groups with a different active router. GLBP is different: one AVG hands out the virtual MAC addresses of all AVFs in its ARP replies, so all routers forward traffic. When VRRP doesn’t fail over as expected, you’ll learn how to troubleshoot VRRP.
DNS and Using a Router as a Host
DNS translates hostnames into IP addresses. It’s distributed and hierarchical: no single DNS server knows every name, so servers query each other, starting at the root name servers. In labs, you often need a host to test pings, telnet, SSH, or HTTP; a Cisco router can play the role of host or server.
Time Synchronization: NTP and PTP
Log messages and certificates are only useful when your devices have the correct time. NTP synchronizes the clocks of your devices with a central time source and uses stratum levels to show how many hops a device is away from an authoritative clock. NTP is accurate to under ten milliseconds; when you need sub-microsecond accuracy, for example in manufacturing or high-frequency trading, you need PTP. When a device won’t synchronize, you’ll learn how to troubleshoot NTP.
IP SLA, BFD and WCCP
IP SLA measures network performance, from a simple ping to delay and jitter for VoIP, and you can combine it with static routes, policy-based routing, and HSRP object tracking to fail over when a path stops working. BFD detects link failures in milliseconds and notifies protocols like OSPF, EIGRP, BGP, and HSRP. WCCP lets a Cisco router redirect web traffic to a proxy server like Squid for caching and filtering.
Frequently Asked Questions
IP services is one of the main topic areas of CCNA 200-301. You should understand DHCP, DNS, NAT and PAT, NTP, SNMP, syslog, and SSH, and be able to configure and verify common ones like NAT, DHCP, and NTP. You also need to know what first hop redundancy protocols like HSRP do. Check Cisco’s current exam topics for the exact list. Start with our introduction to DHCP.
All three give your hosts a virtual default gateway that moves to another router if the active one fails. HSRP and GLBP are Cisco proprietary, and VRRP is an open standard. HSRP and VRRP use one active router per group, while GLBP shares the load between several routers behind one virtual IP address. If all your gateways are Cisco, HSRP is a common choice; with mixed vendors, use VRRP. Learn about gateway redundancy.
Not much. GLBP is Cisco proprietary, and many networks get load sharing in other ways, for example with multiple HSRP groups, or with vPC or switch stacks where both switches forward traffic. You’ll still find GLBP in some Cisco networks and in study material, so it’s worth understanding how it works. Learn how GLBP works.
Not really. NAT was invented to save IPv4 addresses, not to protect hosts. PAT does block unsolicited traffic from the Internet as a side effect, because the router has no translation for it, but it doesn’t inspect anything, and with static NAT or port forwarding, traffic from the Internet reaches your inside host just fine. To secure your network, use a stateful firewall. Read our introduction to firewalls.
Usually not. NAT was mainly invented to save IPv4 addresses, and IPv6 has so many addresses that every device can have a globally unique one. Without NAT, you protect your hosts with a stateful firewall instead. Translation between IPv6 and IPv4, like NAT64, still exists for networks that need to reach IPv4-only hosts. Learn how NAT and PAT work.
IP services are the supporting protocols of an IP network, like ARP, DHCP, DNS, NAT, first hop redundancy protocols, and NTP. They don’t route packets themselves, but hosts and network devices need them to get an address, reach the Internet, and keep a working default gateway.
NAT translates one IP address into another, for example a private address into a public address. PAT also translates port numbers, so many hosts can share a single public IP address at the same time. PAT is what most networks use for Internet access. Learn how NAT and PAT work.
A host without an IP address broadcasts a DHCP discover message. The DHCP server replies with an offer, the host requests the offered address, and the server confirms with an acknowledgment. When the DHCP server is in another subnet, the router needs to be a DHCP relay agent to forward the broadcast. Read our introduction to DHCP.
IP SLA measures network performance with probes like ICMP echo, or delay and jitter for VoIP traffic. You can combine it with object tracking, so a static route, policy-based route, or HSRP priority changes when a probe fails. Learn how to configure IP SLA.