CCNA: Networking Fundamentals

• OSI model layers

  (Fundamentals) Physical, Data Link, Network, Transport, Session, Presentation, Application - layers 1 through 7.

• Encapsulation order

  (Fundamentals) Data becomes a segment, then a packet, then a frame, then bits on the wire.

• TCP vs. UDP

  (Transport) TCP is connection-oriented with acknowledgment and retransmission; UDP is connectionless with lower overhead and no delivery guarantee.

• TCP three-way handshake

  (Transport) SYN, SYN-ACK, ACK establishes the connection before data transfer.

• MAC address

  (Layer 2) A 48-bit hardware address; the first 24 bits are the vendor OUI. Used only within a broadcast domain.

• Switch MAC address table

  (Layer 2) Maps learned source MAC addresses to ports. Unknown unicast frames are flooded to all ports except the ingress port.

• Collision domain vs. broadcast domain

  (Layer 2) Each switch port is its own collision domain; each VLAN is its own broadcast domain.

• VLAN

  (Layer 2) A logical segmentation of a switch into separate broadcast domains. Traffic between VLANs requires a router or layer 3 switch.

• Access port vs. trunk port

  (Layer 2) An access port carries one untagged VLAN to an end device; a trunk carries multiple VLANs using 802.1Q tags.

• Native VLAN

  (Layer 2) The one VLAN sent untagged across an 802.1Q trunk. It must match on both ends.

• Spanning Tree Protocol

  (Layer 2) Prevents layer 2 loops by blocking redundant paths. The switch with the lowest bridge ID becomes root.

• PortFast

  (Layer 2) Moves an access port directly to forwarding, skipping listening and learning. Only for ports facing end devices.

• EtherChannel

  (Layer 2) Bundles multiple physical links into one logical link for bandwidth and redundancy. Negotiated by LACP or PAgP.

• IPv4 private ranges

  (IP) 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Not routable on the public internet.

• Subnet mask

  (IP) Separates the network portion from the host portion of an address. A /24 leaves 8 host bits.

• Usable hosts formula

  (IP) Two to the power of the host bits, minus two for the network and broadcast addresses.

• CIDR and VLSM

  (IP) CIDR removes classful boundaries; VLSM allows different mask lengths within the same network to reduce waste.

• Default gateway

  (IP) The router address a host sends traffic to when the destination is outside its own subnet.

• ARP

  (IP) Resolves a known IPv4 address to an unknown MAC address using a broadcast request and a unicast reply.

• DHCP DORA

  (Services) Discover, Offer, Request, Acknowledge - the four messages that lease an address.

• DHCP relay (ip helper-address)

  (Services) Forwards client broadcasts to a DHCP server on another subnet, since broadcasts do not cross routers.

• DNS

  (Services) Resolves names to IP addresses. Uses UDP port 53 for queries and TCP 53 for zone transfers and large responses.

• NAT vs. PAT

  (Services) NAT maps addresses one to one; PAT (overload) maps many private addresses to one public address using port numbers.

• IPv6 address length

  (IPv6) 128 bits, written as eight groups of four hex digits. Leading zeros can be dropped and one run of zeros replaced by a double colon.

• IPv6 link-local address

  (IPv6) Begins with FE80::/10, is automatically configured, and is only valid on the local link.

• SLAAC

  (IPv6) Stateless address autoconfiguration - a host builds its own address from a router advertisement prefix.

• Routing table longest prefix match

  (Routing) A router forwards using the most specific matching route, regardless of protocol or metric.

• Administrative distance

  (Routing) Trustworthiness of a route source when protocols conflict. Connected 0, static 1, EIGRP 90, OSPF 110, RIP 120.

• Static vs. dynamic routing

  (Routing) Static routes are manually configured and do not adapt; dynamic protocols exchange updates and reconverge automatically.

• Default route

  (Routing) 0.0.0.0/0 - the route used when no more specific route matches.

• OSPF areas

  (Routing) A link-state protocol organized into areas; all areas must connect to the backbone, area 0.

• OSPF neighbor requirements

  (Routing) Matching area ID, hello and dead timers, subnet, authentication, and stub flags.

• OSPF cost metric

  (Routing) Based on interface bandwidth - reference bandwidth divided by interface bandwidth.

• First hop redundancy (HSRP)

  (Availability) Two or more routers share a virtual IP so hosts keep a single default gateway if one router fails.

• Standard vs. extended ACL

  (Security) Standard ACLs filter on source address only; extended ACLs filter on source, destination, protocol, and port.

• ACL implicit deny

  (Security) Every access list ends with an implicit deny any. At least one permit statement is required.

• Port security

  (Security) Restricts which MAC addresses may use a switch port. Violation modes are protect, restrict, and shutdown.

• DHCP snooping

  (Security) Marks ports trusted or untrusted and drops DHCP server replies arriving on untrusted ports.

• AAA

  (Security) Authentication verifies identity, authorization determines permissions, accounting records activity.

• SSH vs. Telnet

  (Security) SSH encrypts the management session; Telnet sends credentials in clear text and should not be used.

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