Analyzing IP Networks – Answers, Exercise 3
Today’s post lists the answers to the previous post’s question, with a few comments, with a place to discuss. Nothing snazzy, but it does hit the fundamentals. Enjoy!
Related links:
- The original list of problems
- Post about how to find the answers
- Deeper info from the ICND1 OCG book
Class, Network ID, and Network Broadcast
The Network ID can be derived from the class A, B, or C DDN value by copying the network octets, and writing a 0 for the rest of the octets. Similarly, the network broadcast address can be found by using the same logic, but writing a 255 instead of 0 for the host octets. Table 2 shows the class for each of the five problems, along with the derived network ID and network broadcast address for each class A, B, or C address.
Table 2: Network IDs and Network Broadcast Addresses
| DDN Value | Class | Network ID | Network Broadcast Address | |
| 1 | 126.0.0.1 | A | 126.0.0.0 | 126.255.255.255 |
| 2 | 192.0.0.1 | C | 192.0.0.0 | 192.0.0.255 |
| 3 | 223.0.0.1 | C | 223.0.0.0 | 223.0.0.255 |
| 4 | 1.1.1.1 | A | 1.0.0.0 | 1.255.255.255 |
| 5 | 123.321.123.321 | N/A | N/A | N/A |
Usable Host IP Addresses
To find the range of IP addresses that can be used by hosts in the (unsubnetted) classful network, just add 1 to the network ID and subtract 1 from the network broadcast address. Table 3 shows the results for these five problems.
Table 3: Ranges of Usable Addresses
| Network ID | Lowest Usable Host Address | Highest Usable Host Address | Network Broadcast Address | |
| 1 | 126.0.0.0 | 126.0.0.1 | 126.255.255.254 | 126.255.255.255 |
| 2 | 192.0.0.0 | 192.0.0.1 | 192.0.0.254 | 192.0.0.255 |
| 3 | 223.0.0.0 | 223.0.0.1 | 223.0.0.254 | 223.0.0.255 |
| 4 | 1.0.0.0 | 1.0.0.1 | 1.255.255.254 | 1.255.255.255 |
| 5 | N/A | N/A | N/A | N/A |
Hi Wendell,
why 123.321.123.321 is marked an N/A?
It seems to me a valid Class A IP address, am i wrong?
Hi Paolo,
It’s easy to overlook, but… check out the 2nd and 4th octet values of “321”. That value is not allowed; it must be between 0-255 inclusive.
Ahahah, shame on me!
I focused only on the first octet.
Thank you for the reply!
aha that last one got me. Careless mistake! Thanks for the practice!
Thanks for pointing this out. The last address looked also valid to me until you specified the range 0-255. Easy to overlook, we could find out later when the value exceed the number in the network mask.
Hi sir,
In your CCNA-Volume 1, page 368, Q# 2, why one of the correct answers is D and not E?
Regards,
Davood
Hi Davood,
I wonder if you weren’t aware of the answers appendix C in the back of the book? From the explanation for this question in the appendix:
The router also adds a route called a local route, which is a route for the interface IP
address with a 255.255.255.255 mask. In this case, that means the router adds a local
route for address 10.1.1.100 with mask 255.255.255.255.
Let me know if that doesn’t answer it for you.
Hi,
CCNA volume 1, chapter 1, Q# 4, answer is B(on one computer), but in page 22, you’ve mentioned ” two computers. What’s the correct answer?
Davood,
CCNA Vol 1, Chap 1, Q#4, the correct answer is B as listed in the book. Not sure why you think answer B is “one computer”, as the text for answer B is “Same-layer interaction”? Same layer interaction occurs in the code that implements the same layer’s functions on two different computers, as described in this question. Hope this helps.
Wendell
What will occur to the default-gateway of a SW, if the networker configure both “ip address dhcp” & “ip default-gateway”? Can he configure both?
Thanks
Hey Davood,
First answer: I’m sure both can be configured, as there is no mechanism to prevent it. The question then is if a default gateway is learned w/ DHCP, does the switch use that value or the configured value? I never tried. I’ve added it to my todo list to try it on a switch or two. I’ll do my best to circle back – may be a few more weeks. If you try it for yourself, let me know.
Hello Mr Odom
in the page 325 Chapter 14: Analyzing Existing Subnets, you wrote:
“The original purpose for the subnet broadcast address was to give hosts a way to send one packet to all hosts in a subnet and to do so efficiently. For example, a host in subnet A could send a packet with a destination address of subnet B’s subnet broadcast address. The routers would forward this one packet just like a packet sent to a host in subnet B. After the packet arrives at the router connected to subnet B, — that last router would then forward the packet to * **all hosts in subnet B***, typically by encapsulating the packet in a data-link layer broadcast frame. As a result, all hosts in host B’s subnet would receive a copy of the packet”
please can you explain me ??? i thought the flooding frames is done only by switches and the routers never broadcast a frame????
and how a router broadcast a packet to all hosts
thank you very much
Let me attempt it without writing several pages worth of reply text. 🙂
Your note “I thought the flooding of frames is done by switches” – that is true. Switch flood frames if the switch does not have a match MAC table entry, or if the frame is an ETHERNET broadcast frame.
Your note “… routers never broadcast a… frame”. My book does explain that concept, and it may need improvement. Regardless, to clarify: routers do not forward IP broadcasts to the 255.255.255.255 local subnet broadcast address. Several protocols send packets to that address, notably DHCP. Routers can open and process the packets but they do not forward those packets unchanged.
For packets sent to some subnet’s broadcast address, yes, those are broadcast addresses, and yes, the router forwards them, so maybe I need to improve the explanation in the book. That said, what really happens? Say the packets flows into R1, then R2, then R3, and then into the subnet. R1 does not flood the packet, but instead routes it like any other packet sent to that subnet. R2 likewise forwards the packet like any other packet to a destination in the subnet. When R3, connected to the subnet, forwards the packet into the subnet, it leaves the destination IP address as is. But as that address is not a unicast IP address, there is no matching MAC address in the ARP table, and can’t be. So, R3 encapsulates the packet in an Ethernet frame, with an Ethernet destination address of ffff.ffff.ffff.
So, the routers do not flood the frame, or even broadcast it all over the IP network. That last router does encapsulate in a LAN broadcast frame so it reaches all hosts in the destination subnet.
Hope this helps.
Wendell