Exam Objective 1.3: Troubleshoot IPv4 address configuration, assignment, and subnetting (public and private).
IPv4 addressing and subnetting is the mathematical backbone of the entire exam — it shows up embedded inside routing, ACLs, NAT, DHCP, and troubleshooting scenarios. This objective specifically tests your ability to recognize misconfigured addresses, wrong masks, address-assignment failures, and subnetting mistakes, and to fix them quickly under exam time pressure.
An IPv4 address is 32 bits long, written as four decimal octets separated by dots (dotted decimal notation), such as 192.168.1.10. Each octet ranges from 0 to 255. The address is logically divided into a network portion and a host portion, and the subnet mask determines where that division falls.
A subnet mask is also 32 bits, written the same way, made up of a contiguous block of 1s (identifying the network portion) followed by a contiguous block of 0s (identifying the host portion). For example, 255.255.255.0 in binary is 11111111.11111111.11111111.00000000 — the first 24 bits are network, the last 8 are host.
CIDR notation expresses the same mask as a slash followed by the number of network bits — 255.255.255.0 is /24, 255.255.255.128 is /25, and so on.
Historically, IPv4 addresses were divided into classes, and while classful routing is obsolete, the ranges are still relevant for identifying default masks and private space:
Class A: 1.0.0.0 to 126.255.255.255, default mask /8
Class B: 128.0.0.0 to 191.255.255.255, default mask /16
Class C: 192.0.0.0 to 223.255.255.255, default mask /24
Class D: 224.0.0.0 to 239.255.255.255, reserved for multicast
Class E: 240.0.0.0 to 255.255.255.255, reserved for experimental use
Private IPv4 address ranges (RFC 1918) — these are not routable on the public internet and are reused inside countless private networks:
10.0.0.0 to 10.255.255.255 (10.0.0.0/8)
172.16.0.0 to 172.31.255.255 (172.16.0.0/12)
192.168.0.0 to 192.168.255.255 (192.168.0.0/16)
Public IPv4 addresses are globally unique and routable on the internet, assigned by regional internet registries. Any address not in one of the three private ranges above (and not otherwise reserved) is public.
Other reserved/special ranges to recognize:
127.0.0.0/8 — loopback (127.0.0.1 is the standard loopback test address)
169.254.0.0/16 — APIPA (Automatic Private IP Addressing) / link-local, assigned automatically when a DHCP client fails to reach a DHCP server
0.0.0.0 — represents "this network" or an unspecified address, also used as the default route designation in routing tables
Exam Alert: Recognizing whether an address falls into public or private space, and recognizing APIPA (169.254.x.x) as a symptom of DHCP failure, are both directly and repeatedly tested.
Subnetting borrows bits from the host portion of an address to create additional, smaller networks. Every time you borrow one more bit for the network portion, the number of subnets doubles and the number of usable hosts per subnet is cut roughly in half.
Key subnetting formulas:
Number of subnets = 2 to the power of (borrowed bits)
Number of usable hosts per subnet = 2 to the power of (remaining host bits) minus 2
The minus 2 accounts for the network address (all host bits are 0, identifies the subnet itself, unusable for a host) and the broadcast address (all host bits are 1, used to reach every host on that subnet, also unusable for a host).
Worked example: Given 192.168.10.0/24, subnet it to support at least 6 subnets.
2^2 = 4 (not enough), 2^3 = 8 (enough) — borrow 3 bits, new mask is /27 (255.255.255.224)
Remaining host bits = 8 - 3 = 5, usable hosts per subnet = 2^5 - 2 = 30
Block size (increment) = 256 - 224 = 32
Resulting subnets: 192.168.10.0/27, .32/27, .64/27, .96/27, .128/27, .160/27, .192/27, .224/27
The "magic number" / block size trick: The block size is 256 minus the value of the interesting octet in the subnet mask. Subnets always fall on multiples of the block size. This is the fastest way to identify subnet boundaries during the exam without writing out binary every time.
VLSM (Variable Length Subnet Masking): Rather than subnetting a network into equal-sized blocks, VLSM allows different subnets to use different mask lengths based on how many hosts each actually needs — for example, a point-to-point WAN link might use a /30 (2 usable hosts) while a LAN segment uses a /24. This avoids wasting address space compared to a single fixed subnet size across the whole network.
Wrong subnet mask — the single most common host-connectivity problem. If two hosts are meant to be on the same subnet but have mismatched masks, one host may believe the other is on a remote network (routing the traffic to its default gateway instead of communicating directly), causing communication to fail or become one-directional.
Address outside the correct subnet range — a host configured with an IP address that doesn't actually belong to the subnet in use on that segment (e.g., configured for 192.168.1.50/24 when the segment is actually 192.168.2.0/24) cannot reach anything, including its own default gateway.
Duplicate IP address — two devices configured with the same IP address on the same network causes intermittent connectivity for both, often accompanied by OS-level "duplicate IP address detected" warnings; behavior is unpredictable and depends on which device answers ARP requests at any given moment.
Incorrect default gateway — if a host's default gateway is misconfigured, wrong, or unreachable, the host can communicate with other devices on its own local subnet but cannot reach anything beyond it.
Missing or incorrect DNS server configuration — this doesn't break IP connectivity itself but breaks name resolution, so the symptom is "I can ping the IP address but not the hostname."
Overlapping subnets — when two different network segments are mistakenly assigned overlapping address ranges (common after a merger, VPN interconnection, or manual misconfiguration), routing becomes ambiguous and traffic may never reach its intended destination.
Static assignment — an administrator manually configures the IP address, subnet mask, default gateway, and DNS servers on a device. Used for servers, printers, and network infrastructure devices where a predictable, unchanging address is required.
Dynamic assignment (DHCP) — a DHCP server automatically leases an IP address (plus mask, gateway, DNS, and other options) to a client for a defined lease period. Used for the vast majority of end-user devices.
Symptoms of DHCP-related IPv4 problems:
A client shows an APIPA address (169.254.x.x/16) — this means the DHCP client could not reach any DHCP server within the timeout period and self-assigned a link-local address as a fallback; the client can talk to other APIPA-addressed devices on the same physical segment but cannot reach anything beyond it, including the internet.
DHCP scope exhaustion — if a DHCP server's address pool runs out of available addresses, new clients requesting a lease will fail to get one and typically fall back to APIPA.
Client stuck with a stale/incorrect address — often resolved with an IP release/renew (ipconfig /release and ipconfig /renew on Windows) to force the client to request a fresh lease.
DHCP relay misconfiguration — if clients are on a different subnet than the DHCP server, a relay agent (ip helper-address on a Cisco router interface) must be configured to forward DHCP broadcasts to the server; if this is missing or points to the wrong server address, clients on that remote subnet will fail to get a lease.
Exam Alert: An APIPA address (169.254.x.x) appearing on a client is one of the most heavily tested "symptom to root cause" mappings on the exam. It always points to a DHCP-reachability problem, never to a manual misconfiguration.
Windows: ipconfig /all shows the IP address, subnet mask, default gateway, DNS servers, and DHCP lease information. ipconfig /release and ipconfig /renew force a new DHCP lease request.
macOS/Linux: ifconfig or the newer ip addr show displays the same category of information — assigned address, mask, and interface status.
Common diagnostic commands regardless of OS:
ping — tests basic Layer 3 reachability to a specific address; ping the loopback (127.0.0.1) first to confirm the local IP stack is functional, then the default gateway, then a remote address.
traceroute / tracert — shows the hop-by-hop path to a destination, useful for identifying exactly where along the path connectivity breaks down.
arp -a — displays the local ARP cache, useful for spotting duplicate IP address conflicts or confirming a device has correctly resolved a neighbor's MAC address.
show ip interface brief — quickly displays every interface's assigned IPv4 address and up/down status, a fast way to confirm an interface has the address you expect and is operational.
show running-config (or show running-config interface X) — confirms the exact IP address and mask configured on an interface.
show ip route — confirms whether a route to a given destination subnet actually exists in the routing table; a missing route explains why a subnet is unreachable even when addressing itself is correct.
show ip dhcp binding — on a router acting as a DHCP server, shows which addresses have been leased to which clients, useful for confirming whether DHCP is actually assigning addresses as expected.
debug ip dhcp server events — shows real-time DHCP transaction activity, useful when troubleshooting why a client isn't receiving a lease (use cautiously in production, since debug commands can be CPU-intensive).
Confirm the assigned address, subnet mask, and default gateway on the affected host (ipconfig /all or the router's show running-config).
Verify the address actually belongs to the correct subnet for that segment, and that the mask matches what every other device on the segment is using.
Check for an APIPA address (169.254.x.x) — if present, the problem is DHCP reachability, not addressing itself; move investigation to the DHCP server, relay configuration, or scope capacity.
Ping the loopback address, then the default gateway, then a known-good remote address, to isolate exactly where connectivity breaks.
Check arp -a for any signs of a duplicate IP address conflict.
On the router, confirm the interface is up/up and correctly addressed with show ip interface brief, and confirm a route to the destination subnet exists with show ip route.
If DHCP is involved, confirm scope utilization and relay (ip helper-address) configuration on the router interface serving that subnet.
Q1. A host is configured with IP address 192.168.5.10 and subnet mask 255.255.255.0. It can reach other devices in the same building but cannot reach anything outside the local subnet. What should be checked first?
A. Whether the subnet mask is a private address
B. The host's default gateway configuration
C. Whether the address is APIPA
D. The DHCP server's lease time
Answer: B. The ability to reach devices on the local subnet but nothing beyond it is the textbook symptom of a missing, incorrect, or unreachable default gateway. Local subnet communication doesn't require a gateway, so it succeeds; anything beyond the local subnet requires the gateway to forward traffic, so it fails.
Q2. A Windows client shows an IP address of 169.254.12.34 with a subnet mask of 255.255.0.0. What does this indicate?
A. The client received a valid lease from a DHCP server on a remote subnet
B. The client is using a private RFC 1918 address correctly
C. The client failed to reach a DHCP server and self-assigned an APIPA address
D. The client is configured with a static public IP address
Answer: C. The 169.254.0.0/16 range is reserved for APIPA (Automatic Private IP Addressing) — a Windows client automatically assigns itself an address in this range when it cannot reach any DHCP server within the expected timeout. It indicates a DHCP-reachability problem, not a normal valid lease.
Q3. Which of the following is a valid private IPv4 address range as defined by RFC 1918?
A. 172.32.0.0 to 172.63.255.255
B. 11.0.0.0 to 11.255.255.255
C. 172.16.0.0 to 172.31.255.255
D. 192.169.0.0 to 192.169.255.255
Answer: C. The three RFC 1918 private ranges are 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255), and 192.168.0.0/16. The other answer choices fall outside these specific ranges and are actually public address space.
Q4. An administrator needs to subnet 10.10.20.0/24 to create subnets that each support at least 25 usable hosts, using the minimum number of subnets necessary. Which subnet mask should be used?
A. /26
B. /27
C. /28
D. /25
Answer: B. A /27 leaves 5 host bits, giving 2^5 − 2 = 30 usable hosts per subnet — enough to meet the 25-host requirement while wasting far fewer addresses than a /26 (62 usable hosts, more than double what's needed).
Q5. Two hosts on the same physical switch segment are unable to communicate, and both display "duplicate IP address" warnings on their local operating systems. What is the most likely cause?
A. A DHCP scope has been exhausted
B. Both hosts were manually or dynamically assigned the same IP address
C. One host is using a private address and the other public
D. The default gateway is misconfigured on both hosts
Answer: B. A duplicate IP address conflict occurs when two devices on the same network end up configured with the identical IP address, whether through manual static misconfiguration or a DHCP server error. This produces intermittent, unpredictable connectivity for both devices as they compete to answer ARP requests for that address.
Q6. Which command on a Cisco router would an administrator use to quickly verify that an interface has the expected IPv4 address and is in an up/up state?
A. show ip route
B. show ip interface brief
C. show ip dhcp binding
D. debug ip dhcp server events
Answer: B. show ip interface brief provides a fast summary of every interface's assigned IPv4 address along with its up/down status for both the physical line and the line protocol, making it the quickest way to confirm addressing and interface state at a glance.