IP addressing is the most important topic of the CCNA. You must know IP addressing inside and out in order to build, modify and troubleshoot a network. The CCNA will test your knowledge in this topic with lots of diagrams. Another core topic to know for the CCNA is routing, of course. This is a network certification which is based around routing packets, over WANs.
Fundamentals
OSI Layer 3 – defines routing and logical addressing.
OSI Network Layer defines: Routing, Logical addressing, Routing protocol, Other utilities such as DNS, DHCP and ARP.
OSI Layer 3 major protocol – IP, connectionless network layer protocol
IP – to route data (packets) from source host to destination host
IP will try to deliver packet but is discarded if cannot be delivered w/ no error recovery.
Routing
Routing – end-to-end logic of forwarding data
Routers discard data-link headers and trailers and extract IP packet.
Routers build routing tables with the use of a routing protocol.
Routing protocols learn about locations of the network layer groups in a network and advertise the groups’ locations.
A routing protocol learns routes and puts those in a routing table. RIP is a routing protocol.
Routing protocol defines the type of packet forwarded, or routed, for example IP is a routed protocol.
Rules for IP addresses:
- IP address in the same group cannot be separated by a router
- IP addresses separated by a router must be in different groups.
Learn how to use Cisco with Train Signal. Cisco is the leader in networking technology, touching every part of the enterprise. Click Here!
Concepts to memorize:
- When a host sends a packet it will send directly to the destination host if the IP address is on the same subnet as host.
- If destination IP address is not on the same subnet it will send packet to the default gateway.
When a router has to decide where to forward packets it will check data-link FCS field to ensure no errors occured, if errors occurred it will discard the frame. If no errors, it will discard old data-link header & trailer. It will then compare the IP packet’s destination IP address to the routing table, and find a route that matches the destination IP address. Helps identify outgoing interface of router and the next-hop router. It then encapsulates the IP packet inside a new data-link header and trailer, appropriate for outgoing interface (PPP or HDLC) and forwards the frame.
IP routing protocol goals
- dynamically learn & fill routing table w/ a route to all subnets in network
- place best route in routing table if more than one route to a subnet is available
- notice when routes in the table are no longer valid, and to remove them from the routing table
- if a route is removed from routing table & another route through another neighboring router is available, to add the route to the routing table
- to add new routes, or replace lost routes, w/ best currently available route as quickly as possible
- to prevent routing loops
Steps of a routing protocol
step 1: each router adds a route to its routing table for each subnet directly connected to the router.
step 2: each router tells its neighbors about all the routes in its routing table, including the directly connected routes and routes learned from other routers.
step 3: after learning a new route from a neighbor, the router adds a route to its routing table, with the next-hop router typically being the neighbor from which the route was learned.
Related posts:
- Layers of the OSI Model
- TCP/IP Model: Networking Basics
- CCNA Notes on the Basics of WANs
- CCNA IP Routing – Administrative Distance
- WAN Link Protocols
