Subnet Mask, WorkSheet Help

Nbr of Bits in Subnet Mask Dec Bin Nbr of bits to Mess with Nbr of Subnets Number of hosts Nbr of Bits in host or Subnet Max Number of Hosts or Subnets
0 0 0000 0000 0 0 254 1 0
1 128 1000 0000 7 0 126 2 2
2 192 1100 0000 6 2 62 3 6
3 224 1110 0000 5 6 30 4 14
4 240 1111 0000 4 14 14 5 30
5 248 1111 1000 3 30 6 6 62
6 252 1111 1100 2 62 2 7 126
7 254 1111 1110 1 0 0 8 254
8 255 1111 1111 0 0 0 9 510
9           10 1,022
Subnet Workspace
128 64 32 16 8 4 2 1
11 2,046
12 4,094
13 8,190
14 16,382
15 32,768
16 65,534
Description Start Finish
Class A 1.x.x.x 126.x.x.x
Class B 128.x.x.x 191.x.x.x
Class C
Private Ranges,  RFC 1918  
  10.x.x.x 10.x.x.x
  172.16.x.x 172.31.x.x
  192.168.x.x 192.168.x.x

**Man is a visual Animal, at least the AD/LDD amongst us. This is a sheet I've used for some time now when Subnet calculators or spreadsheets are not available, like for instance, when you are taking an exam.  You can sketch this out on a sheet of paper before starting the exam, and can use it for reference.  CCNA exams are often heavy on Subnet calculations.  do not underestimate the amount of questions on this subject. Do not go into the exam unless you can easily get the correct answers on all practice IP addressing and subnet questions. 


Application Port numbers


Port Number Protocol
FTP Data 20 TCP
FTP Control 21 TCP
Telnet 23 TCP
DHCP 67/68 UDP
POP3 110 TCP




Router memory types

Memory Type Use
ROM Bootstrap loader and Troubleshooting.
Flash Contains IOS
NVRAM Holds router configuration (Startup configuration)
RAM Working Memory and running configuration.


Command Line Interface Modes

Mode Description
User mode First logged on, good for troubleshhooting and displaying system information
Enable or Privileged Mode Similar to root access in Unix.  Allows entrance to specific change modes
Global Configuration Mode. Direct config commands into router memory
Interface Config Mode Change options for Ineterfaces, E0, S3, etc.
Line Config Mode Vty, console, tty or Async change mode.
Router Config mode change the actual routing engine parameters.



Different ways of booting a router.

Using the Config-register command

0x2102 Normal boot
0x2100 Boot to Rom Monitor
0x2101 RXBoot mode (Limited IOS image help in ROM (EPROM)
0x2142 Bypass NVRAM (Password Recovery)

Boot system command

Router Boot System Comands

No boot Looks first in flash, then broadcasts loking for TFTP server, IOS in ROM and finally ROM Monitor
Boot system ROM IOS from ROM is loaded
Boot system Flash filename Boot from a a specific filename in Flash
Boot system TFTP filename 192.x.x.x IOS is loaded from a TFTP server
Note:  Multiple boot commands canb eu sed and they are processed in the order with which they are e ntered into the router.  

Routing Protocols

Classless N Y N Y Y
Sends Masking Info N Y N Y Y
Supports VLSM N Y N Y Y
Routing Summaries N N N Y Y
Auto Summarization Y Option Y Option Option
Default Update Timer 30s   90s    
Default hold down timer 180 N/A 280 N/A N/A
Infinite Metric 16   4,294,967,295    
Sends out entire table Y Y Y N N
Default Administrative Distance 120 120 100 90 110
Metric Based On Hop Count Hop Count Bandwidth/Delay Hybrid Bandwidth/Delay Hybrid Link State
Public Standard Y Y N N Y
Algorithm Y N DUAL DSPF
Loop Prevention *1 *1 *1 2 3
*1--Hold Down Timer & Split Horizon *2--DUAL and feasible successors *3--Dijkstra SPF and topology knowledge


Cabling standards

Twisted Pair Cable Categories (Copper)
UTP Category Max Speed Description
1 N/A Analog telephones
2 4 mbps Original Token Ring Spec
3 10 Mbps Standard for phones and ethernet
4 16 Mbps The last Token Ring standard before obsolesence
5 1 Gbps Typically installed from yr 2000 onwards
Cat 5E 1 Gbps More $ than Cat 5, but less errors
Cat 6 1 Gbps Very expensive, but with 10Gig support in the future.

UTP Ethernet Cable Pinouts.

RJ-45 8 Position Straight through Cable
1  --> 1
2 --> 2
3 --> 3
6 --> 6

J-45 8 Position Cross Over Cable
1  --> 2
2 --> 1
3 --> 6
6 --> 3


Access Lists

IP Standard 1-99
IP Extended 100-199
Ethernet Type Code 200-299
DecNet 300-399
XNS 400-499
Extended XNS 500-599
Appletalk 600-699
Ethernet Address 700-799
IPX Standard 800-899
IPX Extended 900-999
IPX SAP 1000-1099
IP Standard (more) 1300-1999 (IOS > 12.0)
IP Extended (more) 2000-2699 (IOS > 12.0)

Access list standard example

router(config)#access-list 10 deny
router(config)#access-list 10 permit any
router(config)#interface ethernet 0
router(config-if)#ip access-group 10 in

Access list extended example

router(config)#access-list 105 deny TCP any host eq SMTP
router(config)#access-list 105 permit IP any any
router(config)#interface serial 0
router(config-if)#ip access-group 105 out

ISDN Reference points and Function Groups

1: Reference points and function groups only apply to BRI. PRI(23B+D T1) simply obeys all the rules of a voice T1 when ordering, installing and interfacing, CSU's, 4 wires, smartjacks, yadda yadda. Reference points and function groups are part of the great ISDN BRI spec because ISDN was gonna be such a ubiquitous system it would need plenty of flexibility to handle different types of user equipment. And everybody knows, BRI was for the end user stuff.

2: Everything else in this section is to explain Function Groups and Reference points as they relate to BRI, so that you can pass your CCNA exam. Nothing else.

3: When you order an ISDN BRI from the phone company, it shows up on your premises as two wires. This fact is very important.

4: Most bloody fascinating is that commercially available ISDN premise equipment in North America requires 4 wires. Not 2, like the phone company gives you. So the first thing required is a little black box to make the signal change from a 2 wire format to a 4 wire format. That little black box is called an NT1 (Network Termination 1 North America) or NT2 (Network Termination 2, the rest of planet earth). The little NTx black box falls under the category of "Function Group". Isn't that intuitively obvious? NT1/NT2 makes a 2 wire ISDN BRI signal from the phone company into a 4 wire signal at your premise. An NT1/NT2 is a device that will work in North America and the rest of planet earth.

5: All ISDN Terminal Equipment falls under one of two Function Group categrories. The categories are TE1, (Terminal Equipment 1) for stuff that is ISDN 4 wire capable, which can connect directly into an NT1/NT2 device and TE2 (Terminal Equipment 2), which cannot. TE2 stuff is like analog phones, analog fax, or even stuff with an RS-232 interface like a dumb terminal, or your toaster or coffe cup. Basically, TE2 is anything that is ISDN ignorant. TE1 stuff is 4 wire ISDN capable, and ISDN aware, like an ISDN telephone, fax machine, or PC with an ISDN card in it. How many people do you know who bought one of those $500.00 TE1 ISDN phones for the home?

6: A TA is a Terminal Adaptor that connects to an NT1 or NT2 device, on the premise side of the NT1/NT2 little black box. The TA takes the 4 wire ISDN signal from the NT adaptor, and converts it to either 2 wire dumb analog phone connection (So you can connect legacy analog phones, or fax machines etc.), or RS-232 for a Com port on a PC or dumb terminal. The TA belongs to the Function Group Category.

7: You now know all about the TA, TE1, TE2, NT1, NT2 and NT1/NT2, and you know how they work and where they fit into the scheme of things. This list comprises of the entire Function Group. simple huh?

8: Now the stupid part, reference points. (Don't forget, you are trying to pass an exam) A Reference Point describes the interface between two Function Group items.

9: Ref Point U. (User) This just means I have two wires coming from the phone company. I need to connect 'em into something that can eat 2 wire ISDN, like an NT1 or NT2 device, or an ISDN U Interface on a Cisco Router. So, Ref Point U = that place in space between the serving CO and the premise Based NTx

10: Ref Point S. (System). Between any NTx device and either a TA or TE1. So, Ref Point S = that place in space which is a 4 wire ISDN aware space between the NTx box and the ISDN aware terminal equipment or ISDN aware Terminal Adapter (TE1 and TA).

11: Ref Point T. (Terminal). This describes a point that sits between an NT1 and NT2 device. Used once in a blue moon, if ever.

12: Ref Point S/T. This is simply a combination interface combining S and T.

13: Ref Point R. (Rate) Without getting too excited, this is simply the reference point describing any interface that comes out from the premise side of a TA. It could be serial RS-232, it could be analog phone lines, it could be a tin can and string, or even a Cisco router FXO interface, depends on the flavor of TA you bought.

14: So there are all the reference points you need to know, S, T, S/T, U and R. The only two that interface to a Cisco ISDN interface are U and S/T. R can interface to a Cisco router, just not on an ISDN interface.