The three services on one sheet
| Service | Transport | v1.1 topic | v2.0 topic | What the topic asks |
|---|---|---|---|---|
| NAT | No port of its own; rewrites IPv4 headers, plus TCP or UDP ports for PAT | 4.1 | 4.3 | v1.1: configure and verify inside source NAT, static and pool. v2.0: NAT and PAT on IOS XE |
| DHCP | UDP 67 (server), UDP 68 (client) | 4.3, 4.6 | 1.7 | v1.1: the role of DHCP, then client and relay. v2.0: troubleshoot client, server and relay |
| DNS | UDP 53; TCP 53 for zone transfers and large answers | 4.3 | 4.4 | v1.1: the role of DNS. v2.0: diagnose A, AAAA, CNAME, MX, NS and PTR records |
Topic numbers per Cisco’s v1.1 and v2.0 topic pages, checked October 29, 2026; summaries in our words. By our estimate, an even split of Cisco’s domain weights across topics, this area is about 3% of v1.1 and about 9% of v2.0.
NAT: four address names and three ways to map them
Cisco’s NAT vocabulary is where points leak away, because every term is a pair of words that each carry meaning. Inside or outside says which side of the router the host lives on. Local or global says from which side you are looking at the address. Fix those two axes and the four terms below fall into place.
The mapping types differ in how long a binding lives and how many hosts share an address. Static NAT binds an inside local address permanently, in both directions, to a single inside global address of its own, the usual choice for a server that outside hosts must reach. Dynamic NAT hands out inside global addresses from a pool on demand, one per inside host while its translation lives; once the pool is empty, the next host simply gets no translation. Port Address Translation (PAT) adds the transport port to the binding, which lets flows from many inside hosts leave through a single global address.
Every NAT setup also marks each interface as inside or outside, and translation only happens for traffic that crosses from one role to the other. When a translation never appears, check those interface roles before anything else. The three mapping types get a full side-by-side on static NAT, dynamic NAT and PAT.
- Inside local
- The address actually configured on the inside host, usually from the RFC 1918 private ranges.
- Inside global
- The public address that stands for the inside host on the outside network.
- Outside global
- The address actually configured on the outside host.
- Outside local
- The address by which the outside host appears to inside hosts. It equals the outside global address unless destinations are translated too.
A DHCP lease that crosses a router
The client starts with nothing
With no address and no known server, the host broadcasts its first message from 0.0.0.0 to 255.255.255.255 (RFC 2131).
The router stops the broadcast
Routers do not forward broadcasts. A relay agent on the interface facing the clients takes the message and sends it on as unicast to the server address you configured.
The server picks a pool by subnet
The relay writes the address of its receiving interface into the giaddr field, and the server uses it to choose the pool for that subnet. A server with no matching pool sends nothing back, so the client just keeps waiting.
The lease arrives with options
The answer carries the address, the lease time and options like the gateway and resolver addresses. Halfway through the lease the client renews by unicast directly with the server.
You confirm it on the client
ipconfig /allon Windows,ip addron Linux andipconfig getpacket en0on macOS show what arrived; the macOS command prints the whole lease, options included. An address in 169.254.0.0/16 means the host gave up and assigned itself a link-local address (RFC 3927).
DNS records the v2.0 list names
| Record | Maps | Typical lookup |
|---|---|---|
| A | A name to an IPv4 address | Which IPv4 address does www.example.com use? |
| AAAA | A name to an IPv6 address | Same question, IPv6 answer |
| CNAME | An alias to its canonical name, which the resolver then looks up | Which real name stands behind shop.example.com? |
| MX | A domain to its mail servers, each with a preference value (lower is preferred) | Where does mail for example.com go? |
| NS | A zone to its authoritative name servers | Who answers for example.com? |
| PTR | An address back to a name, under in-addr.arpa or ip6.arpa | Which name belongs to 192.0.2.25? |
Record definitions per RFC 1035 (A, CNAME, MX, NS, PTR) and RFC 3596 (AAAA). A name that holds a CNAME may hold no other records (RFC 1034), which is why a bare domain cannot simply be an alias.
The NAT, DHCP and DNS capture
Before reading the options, decide which role the router plays in the frame: DHCP server, relay, client, or the NAT boundary. The practice test, filtered to NAT, DHCP and DNS, runs this area inside the mixed session.
Practice frames written by our editors for study, not Cisco items. The 200-301 exam itself mixes multiple choice, drag-and-drop and lab-style items.
Frame 001 of 015 · 0 answered · 0 ok
Keys
1–5 or A–E pick an option (on choose-N frames they toggle; then Check answer) · ← → previous / next frame · ↑ ↓ move in the frame list
- Frame 001 of 015
When multiple DNS servers are configured via the ‘ip name-server’ command on a Cisco router, what is the expected behavior if the primary DNS server becomes unresponsive?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
The local host table only holds names defined with ‘ip host’; it does not replace the server list.
Note on B
Correct: IOS tries the name servers in the order configured, so an unanswered query moves on to the next one.
Note on C
The lookup only fails after every configured server has been tried without an answer.
Note on D
IOS does not retry the first server forever; after its timeout it moves to the next server in the list.
Best answer: B · why
You can list up to six servers with ip name-server. IOS queries them in order: if the first does not answer before the timeout, the next server is tried. Names defined locally with ip host are checked first. A lookup fails only when no server answers.
- Frame 002 of 015
What does the ‘ip dhcp excluded-address 192.168.1.1 192.168.1.10’ command accomplish?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Correct: addresses .1 through .10 are kept out of every pool, so the server never leases them and they stay free for gateways and servers.
Note on B
Tying an address to one MAC is a manual binding in a host pool; excluded addresses are simply never handed out.
Note on C
A pool is created with ‘ip dhcp pool NAME’ and a network statement; this command defines no pool.
Note on D
Excluded addresses are not given to clients at all, neither statically nor dynamically.
Best answer: A · why
ip dhcp excluded-address takes a single address or a start and end address and removes that range from all DHCP pools on the router. It is used for addresses set by hand, such as the default gateway, servers and printers. A fixed address for one client is a manual binding instead.
- Frame 003 of 015
Which command is used to verify the active NAT translations on a Cisco router?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
IOS has no ‘show ip nat configuration’ command; NAT settings appear in the running configuration.
Note on B
This filter shows the NAT lines of the configuration, not the translations currently in the table.
Note on C
Statistics give totals, hits, misses and pool use, but not the individual translation entries.
Note on D
Correct: it lists each active entry with inside global, inside local, outside local and outside global addresses.
Best answer: D · why
show ip nat translations prints the NAT table: protocol, inside global, inside local, outside local and outside global, with ports for PAT entries. Add verbose for timers. show ip nat statistics gives counts and pool use, and clear ip nat translation * empties the dynamic entries.
- Frame 004 of 015
Which command configures a router to act as a DHCP relay agent, forwarding DHCP broadcasts from clients to a DHCP server at 10.1.1.254?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
This is not how IOS relays DHCP; a relay needs a helper address on the client-facing interface.
Note on B
Correct: on the interface facing the clients, it turns DHCP broadcasts into unicasts sent to 10.1.1.254.
Note on C
‘ip forward-protocol’ chooses which UDP ports a helper forwards; it does not name a server.
Note on D
IOS uses ‘ip dhcp relay’ only for relay-agent options such as option 82; the server target is set with a helper address.
Best answer: B · why
DHCP Discover and Request messages are broadcasts, which a router does not forward. ip helper-address 10.1.1.254 on the interface that faces the clients makes the router relay them as unicasts to the server, with the interface address as the gateway address so the right pool is chosen. By default it also relays a few other UDP services, such as DNS and TFTP.
- Frame 005 of 015
You have configured a DHCP server on a router and now you need to verify that the DHCP pool and its details are correctly configured. Which command should you use to check the details of the DHCP pool?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Correct: it shows each pool with its network, utilization, leased-address count and the next address to hand out.
Note on B
The running config shows the pool commands you typed but not how the pool is actually being used.
Note on C
This shows interface addresses and status; it says nothing about DHCP pools.
Note on D
Bindings list leases already given to clients, not the settings of the pool itself.
Best answer: A · why
show ip dhcp pool displays each configured pool: the subnet, utilization, leased and total addresses, and the next address to offer. show ip dhcp binding lists individual leases, show ip dhcp conflict lists addresses found in use, and the running configuration shows only the commands.
- Frame 006 of 015
Which command configures a router interface to obtain its IP address via DHCP?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Correct: entered on an interface, it makes the router a DHCP client that requests its address, mask and often a default route.
Note on B
This creates a DHCP server pool on the router; it does not give an interface an address.
Note on C
A helper address relays clients’ DHCP broadcasts to a server; the interface keeps its own static address.
Note on D
Excluded addresses stop a local server from leasing them; the router itself gets no address that way.
Best answer: A · why
ip address dhcp in interface configuration turns the interface into a DHCP client, which is common on an internet-facing interface. show dhcp lease or show ip interface brief confirms the address. ip dhcp pool and ip dhcp excluded-address belong to the server role, and ip helper-address to the relay role.
- Frame 007 of 015
Which command is used to verify the address bindings on a Cisco router configured as a DHCP server?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
This is not the IOS server command for leases; database agents appear under ‘show ip dhcp database’ instead.
Note on B
IOS has no ‘show ip dhcp clients’ command; leases are read from the binding table.
Note on C
IOS has no ‘show ip dhcp leases’ command; the binding table already holds each lease and its expiry.
Note on D
Correct: it lists each leased IP address with the client ID or MAC, the lease expiry and the binding type.
Best answer: D · why
On an IOS DHCP server, show ip dhcp binding lists every lease: IP address, client ID or hardware address, lease expiration and type (automatic or manual). clear ip dhcp binding removes entries, show ip dhcp pool gives pool use, and show ip dhcp conflict lists addresses found already in use.
- Frame 008 of 015
What type of message is a UDP packet that a DHCP server sends to a client to acknowledge the receipt of a client’s request?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
The Offer proposes an address in reply to Discover; the client has not requested anything yet.
Note on B
Discover comes from the client as a broadcast looking for servers; servers never send it.
Note on C
Correct: the DHCPACK confirms the client’s Request and commits the address, lease time and options.
Note on D
A NAK also answers a Request, but it refuses it, for example when the requested address does not fit the subnet.
Best answer: C · why
DHCP runs Discover, Offer, Request, Acknowledge (DORA) over UDP 67 and 68. The client broadcasts Discover, servers reply with Offer, the client broadcasts a Request for one offer, and the chosen server confirms with ACK. A NAK tells the client to start over.
- Frame 009 of 015
A network administrator has configured NAT using a predefined pool to allow inside hosts to access the internet. Which command displays overall NAT statistics including hits, misses, active translations, and pool utilization details?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
There is no such show command in IOS; the NAT settings live in the running configuration.
Note on B
Translations show individual entries, not totals for hits, misses and pool allocation.
Note on C
Correct: it reports active translations, hits and misses, inside and outside interfaces and per-pool allocation.
Note on D
A configuration filter shows only the NAT commands, never live counters.
Best answer: C · why
show ip nat statistics summarizes NAT: total active translations (static, dynamic, extended), inside and outside interfaces, hits and misses, and for each pool the total, allocated and missed addresses. Misses climbing while the pool shows full allocation is the classic sign of an exhausted pool without overload.
- Frame 010 of 015
What service is used to assign IP addresses automatically to devices on a network?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
NAT changes addresses in packets as they cross a router; it does not assign addresses to hosts.
Note on B
Syslog collects log messages from devices; it plays no role in addressing.
Note on C
NTP keeps clocks in step; it hands out time, not IP addresses.
Note on D
Correct: a DHCP server leases addresses, masks, a gateway and DNS servers to clients automatically.
Best answer: D · why
DHCP leases an IP address to a host for a set time, together with options such as the subnet mask, default gateway and DNS servers. A Cisco router can be the server (ip dhcp pool), a relay (ip helper-address) or a client (ip address dhcp).
- Frame 011 of 015
What is the function of the command “ip nat inside source” in a Cisco router?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Viewing the table is ‘show ip nat translations’; ‘ip nat inside source’ is a configuration command.
Note on B
Correct: it defines how inside source addresses are translated, statically or through an ACL with a pool or an interface.
Note on C
Static routes use ‘ip route’; NAT commands do not add routes.
Note on D
DHCP is enabled with ‘ip dhcp pool’ and related commands, not with a NAT statement.
Best answer: B · why
ip nat inside source is the global command that defines source NAT for traffic from inside interfaces: static (ip nat inside source static), dynamic (list ACL pool NAME) or PAT (list ACL interface NAME overload). The interfaces are also marked with ip nat inside and ip nat outside.
- Frame 012 of 015
Which NAT command is used to configure a router to translate all internal private addresses to a single public IP address?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
Outside source NAT translates the addresses of outside hosts, the opposite direction from this goal.
Note on B
Correct: overload on the outside interface address is PAT, so all inside hosts share one public IP, told apart by port number.
Note on C
This only defines a pool; without a matching inside source rule it translates nothing.
Note on D
A static entry maps one private address to one public address, not all inside hosts.
Best answer: B · why
Port Address Translation (NAT overload) maps many inside local addresses to one inside global address and keeps sessions apart by source port. ip nat inside source list 1 interface g0/1 overload uses the outside interface’s own address; the ACL selects which inside sources are translated.
- Frame 013 of 015
Which acronym describes a system responsible for resolving and mapping domain names to IP addresses in a network?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
HTTP carries web pages; it relies on name resolution but does not provide it.
Note on B
SMTP transfers email between mail servers and does not resolve names.
Note on C
Correct: DNS maps names such as www.example.com to addresses through A and AAAA records, on UDP and TCP port 53.
Note on D
NTP synchronizes device clocks and has nothing to do with names.
Best answer: C · why
The Domain Name System resolves host names to IP addresses (A records for IPv4, AAAA for IPv6) and back (PTR). Queries normally use UDP 53, while large answers and zone transfers use TCP 53. On IOS, ip name-server sets the servers and ip domain-lookup enables resolution.
- Frame 014 of 015
Which command is used to configure a NAT pool for translating inside addresses to a range of public IP addresses from 203.0.113.10 to 203.0.113.20?
Pick an option. The verdict shows here; open the note under any option to see why.
Note on A
IOS has no ‘ip nat translation range’ command; public address ranges are defined with ‘ip nat pool’.
Note on B
This rule uses an existing pool for dynamic NAT; by itself it does not define the range.
Note on C
There is no ‘ip nat address-range’ keyword; the range and mask belong in a pool definition.
Note on D
Correct: it names the pool, gives the first and last public address and the mask, ready for an inside source rule.
Best answer: D · why
ip nat pool NAME, then the start and end address and a netmask (or prefix-length), defines the public range for dynamic NAT. The pool is then tied to inside hosts with ip nat inside source list ACL pool NAME, and the interfaces are marked ip nat inside and ip nat outside.
- Frame 015 of 015Choose 2
Which two statements are true about Network Address Translation (NAT)? (Choose two.)
Choose 2 options, then Check answer. Open the note under any option to see why.
Note on A
NAT needs no authentication server; the router translates addresses on its own.
Note on B
SAE is the WPA3 Wi-Fi handshake; NAT offers no encryption or eavesdropping protection.
Note on C
Correct: with PAT, many private hosts share one public address, which saves scarce IPv4 space.
Note on D
Correct: outside hosts see only the public address, so inside addresses stay hidden, though NAT is no substitute for a firewall.
Best answers: C and D · why
NAT was created to stretch the IPv4 address space: PAT lets a whole private network share one public address by tracking source ports. A side effect is that internal addresses are not visible from outside. That hides the topology but is not a security control on its own; filtering still needs ACLs or a firewall.
Concepts in this study area
- Static NAT, dynamic NAT, PATOne-to-one, pool and overload, plus the four NAT address terms.