ICND1 100-101 Study Progress

23 May


I’m starting to see the fruits of an aggressive study plan. Here we are, May 23rd, roughly two weeks until test time and I am nearly on track.

Part I: Networking Fundamentals
Part II: Ethernet LANs and Switches
Part III: Version 4 Addressing and Subnetting (Be done by May 11th and practice subnetting)
Part IV: Implementing IP Version 4 (Be done by May 18th and practice show commands)
Part V: Advanced IPv4 Addressing Concepts
Part VI: IPv4 Services (Be done by May 26th, decide if I want to skip IPv6, Review OSPF and practice more advanced subnetting)
Part VII: IP Version 6
Part VIII: Final Review (Be here by June 1st and have taken a practice exam to decide what areas to review)

I got off to a rocky start with an older 2008 version of the book. Fortunately my study buddy had purchased the correct book instead of borrowing an old one. I had gotten two chapters into the old book and before I started to really get into the newer edition that took a week to recieve. I decided to take a practice test early on. The test is very configurable. I chose study mode for 45 questions and limited myself to 90 minutes with a small chunk of whiteboard. I also decided to exclude any IPv6 questions from this first stab.

After two chapters and a couple videos on subnetting I was able to get a 600 which is 200 points away from passing. This was on the practice test that came on the CD in the book. The higher layer concepts I did quite well on where as the lower layer concepts such as Routing, WANs, ACLs and any kind of IOS commands and configuration questions I did very poorly on. Subnetting seems to get a lot of attention either directly, or indirectly and I was sitting at about 50% or less on that.

What is subnetting?

Don’t listen to me, I’m not an expert, but I don’t think there are many good explanations of this out there. A lot of people go way deep and off on tangents to frequently. Here is my overview of what I understand are important subnetting concepts for ICND1.

IP Address = 32 bits = 4 Octects = 4 bytes

Each byte can store 256 possible combinations of 1s and 0s. So lets represent in binary, 00001001.00000000.00000000.00000001

See, that is 32 bits in an IP address.

The second concept we need to understand is the netmask. Picture a mask you might put on your face. A very thick mask you won’t be able to see much. A thin mask you might be able to see a lot.

Take that concept and apply it to this very common netmask, or 11111111.11111111.11111111.00000000

Out of all the possible combinations that is a pretty thick mask so I can only see a small number of hosts with that mask. If you combine the IP & netmask, you will be able to see IP address from or 256 possible hosts.

And there you have it, networking. Wait, what was I talking about? Ah yes, SUBnetting.

Subnetting takes those 256 possible hosts and divides them into smaller networks. If I needed several separate networks and only 18 hosts per network I could split that network into smaller chunks. If I want to see fewer hosts in my network I need a thicker, or higher number mask.

Pulling up the /24 mask again, 11111111.11111111.11111111.00000000 you will see it is /24 because there are 24 1s or network bits and 8 0s or host bits.

In our problem, we need at least 18 IP address options for hosts. For this we will use 0s. How many 0s will we need? Less than 8 for sure because that gave me 256 options. But how many less?

The powers of 2 come in handy for any binary math. There are 2 possible values for each bit, 0 or 1. With 2 bits there are 4 possible values, 00, 11, 10, 01. That isn’t going to get me to at least 18 hosts. This could take a while and for the ICND1 test you need to subnet in 15 seconds. Yikes!

In comes the cheat sheet.


Memorize this formula to go with the table: Possible hosts on a network = 2^h – 2

Each network supports 2^h ip addresses, however 1 ip address is used for the network id and another is used for the broadcast address, hence the minus 2 part.

I don’t suggest just memorizing the table. I would suggest understanding how to generate the table. Start from the top right and do your powers of 2 up to 128. 2^0 = 1, 2^1 =2 2^2=4 … 2^7=128

Next is the second row, the decimal mask. Take 256 – the h row to get the decimal mask row.

Next is the last 2 octets of cidr notation. This is simply a count of 1s in the binary representation of the mask. Remember 1s are the network bits and 0s are the host bits.

Once we have this table we can solve our problem, subnet in a way that supports at least 5 networks and at least 18 hosts in each network.

Start this question with the important number h, or 18.

Go to the table and find the h value that supports at least 18 hosts, which is 32.

Go down to the decimal notation .224 and we know that we can support at least 18 hosts with a decimal mask of

Next we can list the network IDs that this mask could possibly create

To figure this out mathematically take 2^n where n = the number of network bits. There are 3 network bits or 1s in the octect we subnetted. We can make 8 networks which is greater than 5 required by the problem. BOOM CAKE!


For the remainder of this post I will be simply typing up my notes from the Wendell Odom Cisco Press Book and some other notes I took watching YouTube videos from a variety of authors which I will link to.


1. Physical – wiring standards, physical topology, bandwidth usage, syncronizing bits
2. DataLink – MAC, Flow Control standards
3. Network – IP, IPX, Switching, Route Discovery, TTL
4. Transport – TCP, UDP, windowing, buffering
5. Session – Netbui
6. Presentation – jpg, encryption, data formatting(ascii, ebcidic)
7. Application – http, smb, smtp, service advertisement, dns

IP Addressing

First Octects
CLASS A – 1-127
CLASS B – 128-191
CLASS C – 192-223

Hub – layer 1 device that simply spams all ports with frames

Rember these things in this order
SEGMENT – includes the tcp ports
PACKET – includes the IP
FRAME – the whole stinking thing with headers and trailers

Encapsulation – IP Packet is a Layer 3 PDU

CHAP2: Fundamentals of Ethernet Lans

UTP – unshielded twisted pair

crossover cable

like devices need crossover cable to switch transmit and receive pins

MAC – 48bits – 24 for OUI

FCS – frame check sequence is at the end of the frame to ensure proper delivery


leased line , service provider
CPE – customer premises equipment
CSU/DSU – channel service unit, data service unit usually on prem and RJ-48
Router-Router communication can occur on serial cables
HDLC – high level data link control
——way of encapsulating frames over WAN
PPP – point to point protocol
MPLS – multi protocol label switching

CHAP4: IPv4 Addressing and Routing

Routing uses L3PDUs
Layer 2 are called frames

IPv4 headers are 20 bytes and include SIP,DIP,Len,offset,chksum,ttl,etc…

CLASS A: 126 networks and 16,777,214 hosts per network
CLASS B: 16,384 networks and 65,534 hosts per network
CLASS C: 2,097,152 networks and 254 hosts per network

Router FWD logic
1. uses FCS to make sure no errors
2. discard old frame header and trailer
3. compare DIP to routing table and find next hop
4. encapsulate

CHAP 5: fundamentals of TCP/IP transport applications

UDP – connectionless

Connection establishment
SYN —->

Connection termination

shutdown – command that turns a port down/down
no shutdown – turns a port up/up (the second up is if the protocol works)

CHAP 8: configuring Ethernet Switching

enable secret mYpass
show history
no shutdown

port security
1. switchport mode access (access or trunk)
2. switchport port-security (enables port security)
3. switchport port-security maximum 2 (allowed macs on port)
4. switchport port-security violation shutdown (action to take)
5. switchport port-security mac-address AAAA:AAAA:AAAA (specifiy allowed macs)
6. switchport port-security mac-address sticky (dynamic learned mac addresses)

CHAP 9: implementing VLANs

ISL = OLD protocol

12bits for VLANID (this is a “shim” in the frame)
how many vlans? 2^12 or 4096
vlanid 1 is default

router on a stick – one physical link to a router instead of two

show vlan brief

(allow port 4 to communicate on vlan id 10)
1. enable
2. configure terminal
3. interface FastEthernet0/4
4. switchport access vlan 10

Layer3 switch does routing …but can’t do this in packettracer :[

Reasons switch prevents VLAN traffic from crossing a trunk
1. removed from allow list
2. vlan doesn’t exist in show config
3. vlan doesn’t exist, been disabled

and some other less important reasons

CHAP 10 Troubleshooting

show cdp neighbors
show interfaces status

“administratively down” means shutdown command was run
err-disabled means port security

vlan troubleshooting
1. identify all access interfaces and their vlans
2. do vlans exist and are they active
3. check allowed vlan list on both ends of the trunk
4. check for trunk/no trunk neighbors

show vlan brief


IPv4 subnetting

One subnet for every:
1. vlan
2. ppp serial link
4. frame relay

VLSM – variable length subnet mask


CHAP 12 analyzing classful IPv4 Networks
CHAP 13 analyzing subnet masks
CHAP 14: analyzing existing subnets

CHAP 15: Operating Cisco routers

Installation steps
1. connect lan ports
2. connect CSU/DSu external
3. connect CSU/DSU internal
4. connect console port to pc using a rollover cable
5. connect power
6. power on

show ip route

show mac address-table

status layer 1/status layer 2
down/down : has not been shutdown but physical layer problem

CHAP 16: configurating IPv4 addresses and routes

1. choose to process frame
-proper mac (is its destination me?)
-no errors (FCS)
2. de-encapsulate packet
3. compare DIP to routing table
-this identifies outgoing interface
4. encapsulate
5. transmit

routers should ignore switch floods not intended for it

large routing tables can cause performance problems

cisco express forwarding
-uses organized tree and other tables to help speed up routing

adding routes can be done via:

1. connected routes
2. static routes
3. routing protocols

cisco will add routes if the interface is IP’d and UP

ROAS 802.1Q trunk


commands to turn on

router ospf 1
network area 0

ospf – open shortest path first – uses link state
OSPFv2 is for IPv4

routing protocol – set of messages, rules and algorithms (RIP, EIGRP,OSPF,BGP)

routed & routable protocol – defines packet structure and addressing (IPv4)

1. learn routing information about ipsubnets from neighboring routers
2. advertise this info
3. if more than 1 route exists, pick best
4. if topology changes, advertise current best route (convergence)

Interior gateway protocol – designed for use inside a single autonomous system
exterior gateawy protocol – BGP

routing algorthims use
1. distance vector
2. advanced distance vector
3. link state (ospf uses this)

RIP is old
IGRP is a little less old

RIP-2 uses hop count and is also old with slow convergence
OSPF is a cost based protocol
EIRGP – cisco proprietary and uses bandwidth and latency
IS-IS – uses link state

0 connected
1 static
20 BGP E
110 OSPF
115 IS-IS
120 RIP
200 BGPI

this will show the database of link state advertisements(LSAs)
show ip ospf database

routers must agree to be neighbors

configuration, this will turn on for any interface that matches 10.0.* because of the wildcards in network command

router ospf
network area 0


Discover – TO FROM

ip helper-address {dhcp server ip} – command for router that enables DCHP servers to sit outside of the subnet by changing SIP&DIP ( Thanks /u/Sprockle )

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Posted by on May 23, 2015 in Network Admin


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