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CCENT ICND1 100-101 Networking Fundamentals Course

Learn networking fundamentals, IPv4 and IPv6, switching, routing, Cisco IOS, wireless, security, NAT, and troubleshooting for the CCENT ICND1 100-101 exam.

Course orientation and certification context

This course introduces the networking knowledge and Cisco IOS skills associated with the CCENT ICND1 100-101 curriculum. It is designed for beginners, aspiring network technicians, and learners with basic computer and operating-system experience.

CCENT was an entry-level Cisco networking certification focused on foundational network installation, operation, and troubleshooting. The ICND1 100-101 exam covered the first part of the traditional Cisco certification path. ICND2 200-101 covered the next level of routing, switching, and network operations, and the two exams together formed an earlier CCNA pathway. Certification names and exam versions can change, so use this course for its enduring technical foundations and verify current exam requirements separately.

These skills apply beyond examinations. Understanding addressing, switching, routing, device configuration, and troubleshooting supports work with enterprise networks, wireless systems, cloud connectivity, Linux networking, firewalls, and VPNs.

Prerequisites and study approach

• Basic computer hardware and operating-system use.
• Experience connecting a computer to a local network or internet connection.
• Basic command-line familiarity is useful but not mandatory.
• Binary and hexadecimal arithmetic will help with IPv4 and IPv6 addressing.
• Combine reading with a simulator or lab, then verify every change with show commands and connectivity tests.

Use the course curriculum to organize study and the course activity to practice.

Networking foundations

A computer network connects devices so they can exchange data and share services such as files, applications, printers, voice, and internet access. An end host is a device that originates or consumes traffic, such as a laptop, server, printer, or phone.

In a client-server model, clients request services from dedicated servers. In a peer-to-peer model, devices can provide services directly to one another. Common topologies include star, extended star, bus, ring, mesh, and hybrid designs. Modern switched LANs commonly use a hierarchical star: access switches connect endpoints, distribution devices aggregate access, and routers or Layer 3 switches connect networks.

Bandwidth is the theoretical capacity of a link. Throughput is the actual delivered rate. Latency is delay, jitter is variation in delay, and packet loss is data discarded before delivery. Voice and video are particularly sensitive to latency, jitter, and loss.

Network models and encapsulation

A network model divides communication responsibilities into layers. This makes protocols easier to design and faults easier to isolate. A protocol data unit (PDU) is the name given to data at a particular layer.

The OSI model has seven layers; the TCP/IP model commonly combines OSI application, presentation, and session responsibilities into one application layer and combines data-link and physical responsibilities into network access. During encapsulation, an application payload receives a transport header, an IP header, and an Ethernet header and trailer. During de-encapsulation, the receiving device removes those headers in reverse order.

• Physical faults: cable, power, signal, or interface issues.
• Data-link faults: VLAN, MAC learning, duplex, trunk, or switching issues.
• Network-layer faults: addressing, masks, routes, or gateways.
• Transport and application faults: blocked ports, failed services, DNS, or authentication.

Ethernet, switching, VLANs, and STP

An Ethernet frame contains source and destination MAC addresses, a type or length field, payload, and a frame check sequence. A MAC address identifies a network interface on the local link. Unicast traffic targets one destination, multicast targets a subscribed group, and broadcast targets all devices in the broadcast domain.

A switch learns the source MAC address of incoming frames and records it with the ingress port. If the destination is known, it forwards the frame only through the associated port. If the destination is unknown or broadcast, it floods the frame within the appropriate VLAN, except back out the ingress port. A switch port is normally its own collision domain. A VLAN defines a separate broadcast domain.

Speed and duplex are commonly negotiated. A mismatch, damaged cable, excessive errors, or a disabled port can cause poor performance or loss of connectivity. Check interface counters and status rather than assuming that an illuminated link proves correct operation.

VLANs and trunks

A VLAN logically separates users even when they share physical switches. An access port carries one VLAN for an endpoint. A trunk carries multiple VLANs between network devices. IEEE 802.1Q adds a VLAN tag to trunk frames. The native VLAN is the VLAN whose frames are transmitted untagged on an 802.1Q trunk; both ends should agree on the native VLAN. Devices in different VLANs need a router or Layer 3 switch for inter-VLAN communication.

enable
configure terminal
vlan 10
 name USERS
interface gigabitEthernet 0/1
 switchport mode access
 switchport access vlan 10
interface gigabitEthernet 0/24
 switchport mode trunk
 switchport trunk allowed vlan 10,20
end
show vlan brief
show interfaces trunk
show mac address-table

Spanning Tree Protocol

Redundant Layer 2 links improve availability but can create loops. Loops cause broadcast storms, duplicate frames, and MAC-table instability because the same source appears to move between ports. Spanning Tree Protocol (STP) creates a loop-free logical topology while retaining a blocked path for failover.

STP elects a root bridge using the lowest bridge ID, which is based primarily on bridge priority and MAC address. Switches select root ports toward the root, designate forwarding ports for segments, and place redundant ports into a blocking state. Traditional STP states include blocking, listening, learning, and forwarding; a disabled port does not participate.

PortFast lets a trusted endpoint port reach forwarding quickly. BPDU Guard protects such an edge port by disabling it if a spanning-tree BPDU arrives. Use these features only on ports intended for end devices, not switch-to-switch links.

show spanning-tree
interface fastEthernet 0/10
 spanning-tree portfast
 spanning-tree bpduguard enable

IPv4 addressing and subnetting

An IPv4 address is a 32-bit value written as four decimal octets, such as 192.168.10.25. A subnet mask divides the address into network and host portions. CIDR notation writes the number of network bits after a slash, such as /24. The network address identifies the subnet, usable host addresses identify interfaces, and the broadcast address reaches all hosts in that subnet.

To subnet, determine the required hosts, choose a prefix with enough usable addresses, identify the block size, and list each network, host range, and broadcast address. VLSM assigns different mask lengths to different LANs or point-to-point links, conserving address space. Classful A, B, and C addressing is historical background; practical design uses classless CIDR and VLSM.

Public addresses are globally routable; private addresses require translation or another controlled connection to reach the public internet. A host compares the destination with its own network using the mask. It sends local traffic directly to the destination MAC. For a remote destination, it sends the frame to the MAC address of its default gateway, usually a router interface.

IPv6 addressing and connectivity

IPv6 uses 128-bit addresses, written as hexadecimal hextets separated by colons, for example 2001:db8:10:1::25/64. Leading zeros in a hextet may be removed, and one consecutive run of zero hextets may be replaced by :: once per address. The prefix length identifies the network portion.

IPv6 hosts can use Stateless Address Autoconfiguration (SLAAC). A router sends Router Advertisements containing a prefix and other network information; the host forms an address and uses Neighbor Discovery to find neighbors and routers. IPv6 does not use ARP. ICMPv6 Neighbor Discovery performs address resolution and related functions.

interface gigabitEthernet 0/0
 ipv6 address 2001:db8:10:1::1/64
 no shutdown
ipv6 unicast-routing
show ipv6 interface brief
show ipv6 route
show ipv6 neighbors
ping ipv6 2001:db8:10:1::25

IPv4 and IPv6 can coexist through dual stack, tunneling, or translation mechanisms. A device may have both address families active at the same time.

ARP, Neighbor Discovery, and traffic flow

For IPv4, ARP maps a local IPv4 address to a MAC address. A host broadcasts an ARP request, the owner replies with its MAC address, and the result is cached. If the destination is remote, the host resolves the default gateway's MAC instead of the remote host's MAC.

At every routed hop, the Layer 2 frame is removed and rebuilt for the next link. The IP source and destination generally remain the same end-to-end, while the source and destination MAC addresses change at each hop. A router decrements the IPv4 TTL or IPv6 Hop Limit. This distinction explains why a packet can retain its destination IP while having different Ethernet addresses on different segments.

Cisco IOS operation and administration

Cisco IOS is the operating system used by many Cisco routers and switches. A console connection provides local CLI access, commonly through a console cable and terminal emulator. The CLI supports abbreviated commands, contextual help with ?, command history, editing keys, and tab completion.

The running configuration is active in memory. The startup configuration is loaded at boot. Save deliberate changes with copy running-config startup-config. IOS images are software files used to boot the device; configuration files contain operating settings.

enable
configure terminal
hostname R1
enable secret StrongSecret
banner motd ^CUnauthorized access prohibited^C
username admin secret LocalSecret
line console 0
 login local
line vty 0 4
 login local
 transport input ssh
interface gigabitEthernet 0/0
 description LAN gateway
 no shutdown
end
show running-config
copy running-config startup-config
show startup-config

Use erase startup-config and reload only when intentionally resetting a device. Before destructive operations, back up the configuration and confirm console access.

Router interfaces and IP configuration

enable
configure terminal
interface gigabitEthernet 0/0
 description USERS-LAN
 ip address 192.168.10.1 255.255.255.0
 no shutdown
interface gigabitEthernet 0/1
 description WAN-LINK
 ip address 192.0.2.1 255.255.255.252
 no shutdown
end
show ip interface brief
show interfaces
ping 192.168.10.25
traceroute 192.0.2.2

An interface must have a suitable address and mask and must not be administratively shut down. “Up/up” generally means the physical line and data-link protocol are operational. A switch management address is usually configured on an SVI, such as interface vlan 10, with ip address and no shutdown; configure ip default-gateway on a Layer 2 switch so it can reach management devices outside its subnet.

Routing fundamentals

A router maintains a routing table containing destination prefixes and next hops or exit interfaces. It selects the most specific matching route using longest-prefix match. If no more specific route exists, a default route can act as the gateway of last resort.

Administrative distance ranks trust between route sources. A metric ranks paths within a routing protocol. A static route can be configured with ip route destination mask next-hop, and a default route with ip route 0.0.0.0 0.0.0.0 next-hop. IPv6 equivalents use ipv6 route.

Routing protocols: RIP and OSPF

Distance-vector protocols learn routes from neighbors and commonly use a distance and direction calculation. RIP uses hop count, has a finite maximum usable path length, and is simple but limited. Link-state protocols build a topology database and calculate paths. OSPF is a link-state protocol that uses cost, forms neighbor relationships, and organizes larger designs into areas. Introductory deployments commonly use one area, area 0.

router ospf 1
 router-id 1.1.1.1
 network 192.168.10.0 0.0.0.255 area 0
 network 192.0.2.0 0.0.0.3 area 0
 passive-interface gigabitEthernet 0/0
show ip protocols
show ip route
show ip ospf neighbor

A passive interface advertises its connected network without sending routing updates or forming neighbors on that interface. Missing routes commonly result from an incorrect network statement, an unreachable neighbor, a passive interface on the wrong link, mismatched parameters, or an interface that is down.

Network Address Translation

NAT changes IP addressing as traffic crosses a router. An inside local address is the private address used inside; an inside global address represents that host externally. Outside local and outside global describe the external destination as seen from inside and outside perspectives. Static NAT maps one inside address to one public address. Dynamic NAT uses a pool. PAT overload maps many private hosts to one public address by using different transport ports.

interface gigabitEthernet 0/0
 ip nat inside
interface gigabitEthernet 0/1
 ip nat outside
access-list 1 permit 192.168.10.0 0.0.0.255
ip nat inside source list 1 interface gigabitEthernet 0/1 overload
ip route 0.0.0.0 0.0.0.0 198.51.100.1
show ip nat translations
show ip nat statistics

For failed translated connectivity, verify inside and outside roles, the NAT match rule, the default route, return routing, and whether translations appear when an inside host generates traffic.

Transport, application protocols, and services

TCP is connection-oriented and provides sequencing, acknowledgments, retransmission, and flow control. Its three-way handshake is commonly described as SYN, SYN-ACK, and ACK. UDP is connectionless and has less overhead; applications use it when speed, simplicity, or application-level recovery is preferred. A socket combines an IP address and a transport port number.

DHCP uses a discover, offer, request, and acknowledgment exchange. A router configured as a DHCP relay forwards client broadcasts to a server on another subnet. DNS translates names into addresses through recursive and authoritative resolution. Secure management uses SSH rather than Telnet because SSH encrypts credentials and session data.

Wireless networking basics

A wireless LAN includes wireless clients, access points, and sometimes a wireless LAN controller that centrally manages access points, policies, radio settings, and mobility. An SSID identifies a wireless network. Channels are radio frequencies or channel ranges; bands such as 2.4 GHz and 5 GHz have different coverage and interference characteristics. Standards differ in speed, channel width, modulation, and supported bands.

Use WPA2 or an equivalent current secure authentication and encryption method, strong credentials, current firmware, sensible channel planning, and appropriate access-point placement. Investigate interference, distance, obstructions, channel overlap, authentication failures, and incorrect VLAN assignment when clients associate but cannot reach network services.

Network security fundamentals

Threats include credential theft, malware, unauthorized access, eavesdropping, denial of service, misconfiguration, and physical tampering. Apply least privilege: administrators and services should receive only the permissions they need.

• Use strong passwords, enable secret, local users, and protected configuration storage.
• Display an appropriate login banner and restrict physical console access.
• Use SSH with keys and controlled VTY lines; avoid Telnet.
• Use switch port security concepts to limit unexpected endpoint MAC addresses.
• Use ACLs to permit or deny packets according to source, destination, protocol, and port.
• Protect management, control, and data planes separately: management protects administration, control protects routing and device decisions, and data protection governs forwarded traffic.

An ACL is evaluated in order, generally stops at the first match, and has an implicit deny at the end unless a permit rule allows the traffic. Test policy changes carefully to avoid blocking management access.

Troubleshooting methodology

A repeatable process is: identify the symptom and scope, theorize likely causes, test the theory, plan a safe correction, implement one controlled change, verify the result, and document the cause and fix.

• Bottom-up: start with power, cables, interfaces, switching, addressing, routing, and services.
• Top-down: start with the application and work toward the physical layer.
• Divide-and-conquer: test a boundary, such as the default gateway, to decide whether the fault is local or remote.

Useful evidence includes show interfaces, show ip interface brief, show mac address-table, show arp, show ipv6 neighbors, show ip route, logs, ping, and traceroute.

If a host reaches its own LAN but not another subnet, check its mask and default gateway, router interface status, routing tables, ACLs, and NAT. If a switch port works physically but access is wrong, check VLAN membership, trunk allowance, MAC learning, port security, and duplex. If redundant links cause broadcasts, inspect the STP root, port roles, and edge-port settings. If SSH fails, verify IP reachability, a local user, domain name, RSA keys, VTY transport settings, and credentials.

Practical lab sequence

1. Small routed IPv4 network: create two LAN subnets, configure two router interfaces and host gateways, then test local and remote reachability.
2. Subnet a small office: divide one address block into user, server, management, and point-to-point networks using VLSM.
3. VLAN segmentation: place user ports in separate VLANs, configure a trunk, verify VLAN and MAC tables, and explain why inter-VLAN routing is required.
4. Loop prevention: connect redundant switch links, identify the root bridge and blocked path, and apply PortFast and BPDU Guard only to an edge port.
5. Dynamic routing: configure a simple RIP or single-area OSPF topology, confirm learned routes, and compare them with a static route.
6. PAT internet access: mark NAT inside and outside interfaces, translate private clients through one public address, and inspect translations.
7. Secure administration: configure a hostname, secret, local user, SSH, VTY restrictions, and saved configuration.
8. End-to-end fault isolation: introduce an incorrect mask, shutdown interface, missing route, or wrong gateway; locate and correct it with show commands, ping, and traceroute.

Exam-relevant notes

• Know the purpose and mapping of OSI and TCP/IP layers and the PDU names.
• Practice subnet calculations until network, host range, broadcast, and gateway identification are quick and accurate.
• Distinguish MAC forwarding from IP routing and understand when a frame is rebuilt.
• Memorize the meaning of access ports, trunks, native VLANs, STP roles, and PortFast protections.
• Be able to read interface, VLAN, MAC, neighbor, NAT, and routing output before changing configuration.
• Understand that a configured address, an enabled interface, a selected route, and a successful application service are separate validation points.