CCNA exam success depends on mastering the fundamentals, and two important fundamentals are knowing exactly what the terms “collision domain” and “broadcast domain” mean.
In this free Cisco tutorial, we’ll take a look at the term “collision domain” and how a collision domain is defined.
A collision domain is an area in which a collision can occur. Fair enough, but what “collision” are we talking about here?
We’re talking about collisions that occur on CSMA/CD segments, or Carrier Sense Multiple Access with Collision Detection.
If two hosts on an Ethernet segment transmit data at exactly the same time, the data from the two hosts will collide on the shared segment.
CSMA/CD exists to lessen the chances of this happening, but collisions can still occur. To lessen the chances of collisions occurring, we may decide to create multiple, smaller collision domains.
Lets say we have four hosts on a single Ethernet segment. The entire segment is a collision domain; any data sent by one of the hosts can collide with data sent by any of the other hosts. We have one collision domain containing four devices.
To create smaller collision domains, we’ll need to introduce some type of networking device into this example.
Hubs and repeaters have their place as far as extending the reach of a network segment and cutting down on attenuation, but these OSI Layer One devices do nothing to define collision domains. We could connect each host into a separate port on a hub (a hub is basically a multiport repeater) and we’d still have one single collision domain with four hosts in it.
The most common and most effective way to create multiple collision domains is to use a switch.
If we connect each of these four hosts to their own separate switch port, we would now have four separate collision domains, each with one host; each switch port actually acts as a single collision domain, making collisions between these four hosts impossible.
Passing the CCNA is all about knowing the details of how things work, and knowing CSMA/CD theory and how to define collision domains is one of the many details youve got to master.
In the next part of this CCNA tutorial, we’ll take a look at broadcast domains, and how defining broadcast domains in the right places can dramatically cut down on unnecessary traffic on your network.
The BSCI exam and CCNP certification requires that you be well versed in the basics of IP Version 6, or IPv6.
If youre new to IPv6, youll quickly learn that its not exactly just two more octets slapped onto an IPv4 address! IPv6 addresses are quite long, but there are two ways to acceptably shorten IPv6 address expression. To pass the BSCI exam, become a CCNP, and get that all-important understanding of IPv6, youve got to understand these different methods of expressing an IPv6 address. My last IPv6 tutorial discussed zero compression; today well take a look at leading zero compression.
Leading zero compression allows us to drop the leading zeroes from every field in the address. Where we could only use zero compression once in an IPv6 address expression, leading zero compression can be used as often as is appropriate. The key with leading zero compression is that there must be at least one number left in each field, even if that remaining number is a zero.
You sometimes see books or websites refer to leading zero compression as “dropping zeroes and replacing them with a colon”, but that explanation can be a little confusing, since the blocks are separated with a colon to begin with. Youre not really replacing the leading zeroes, youre dropping them.
Lets look at an example of leading zero compression. Taking the address 1234:0000:1234:0000:1234:0000:1234:0123, we have four different fields that have leading zeroes. The address could be written out as it is, or drop the leading zeroes.
Original format: 1234:0000:1234:0000:1234:0000:0123:1234With leading zero compression: 1234:0:1234:0:1234:0:123:1234
Theres no problem with using zero compression and leading zero compression in the same address, as shown here:
Original format: 1111:0000:0000:1234:0011:0022:0033:0044
With zero and leading zero compression: 1111::1234:11:22:33:44
Zero compression uses the double-colon to replace the second and third block of numbers, which were all zeroes; leading zero compression replaced the “00” at the beginning of each of the last four blocks. Just be careful and take your time with both zero compression and leading zero compression and youll do well on the exam and in the real world. The keys to success here are remembering that you can only use zero compression once in a single address, and that while leading zero compression can be used as often as needed, at least one number must remain in each field, even if that number is a zero.