IP Address Rejection

[AthertonB9]

Hi, there I am currently studying CCNA 200-301, I would like to know what makes a valid/invalid (accepted/rejected) ipv4 address. In the scenario below the first address is accepted and the last two are rejected and described as having a bad mask. Along with this reference "because the subnet broadcast address cannot be used as an interface IPv4 address". Can someone explain to me what this means?

[Excerpt from CCNA 200-301 Network Simulator]

In this exercise, you begin by examining three IPv4 addresses and masks. Each pair is a candidate to be configured on Router R1's GigabitEthernet0/0 interface. The question: Will R1 reject the IPv4 address/mask combination as invalid for some reason? If rejected, what is the reason?

After you make your prediction in each case, you will configure the three IPv4 addresses to see how the router reacts. The IPv4 addresses to consider for this lab are as follows in Table 1:

10.100.2.255	255.255.254.0
10.100.2.255	255.255.255.0
10.100.2.255	255.255.255.128

Examine the three IPv4 addresses and masks in Table 1. Consider whether it is a valid unicast IPv4 address and therefore will not be rejected when configured on a router interface. Steps 3-5 will allow you to select whether each row of Table 1 will be Accepted or Rejected; when complete click the Submit button.

10.100.2.255

255.255.254.0

• The number 10.100.2.255, with mask 255.255.254.0, is a valid IPv4 address. It is in a subnet whose subnet ID is 10.100.2.0 with subnet broadcast address 10.100.3.255 (Accepted)

10.100.2.255

255.255.255.0

• Use the   

  IP address 10.100.2.255 255.255.255.0 interface 

  subcommand. In this case, the number 10.100.2.255, when using mask 255.255.255.0, is a subnet broadcast address in a subnet with subnet ID 10.100.2.0. Router R1 rejects this 

  IP address 

  command because the subnet broadcast address cannot be used as an interface IPv4 address. (Rejected)

10.100.2.255

255.255.255.128

• Use the

  IP
   address 10.100.2.255 255.255.255.128 interface 

  subcommand. In this case, the number 10.100.2.255, when using mask 255.255.255.128, is a subnet broadcast address in a subnet with subnet ID 10.100.2.128. Router R1 rejects this 

  IP address 

  command because the subnet broadcast address cannot be used as an interface IPv4 address. (Rejected)

[AthertonB9]

192.168.1.5 / 29  255.255.255.248   (255 = 8 bits 248 = 6 bits)

 248- 252 = 8

0------5---------8  (0 is closer to 5 than 8 is)

Subnet ID 192.168.1.0

First Usable Address: 192. 168.1.1(+ 1 subnet ID)

Last Usable Address: 192.168.1.6(-1 Broadcast Address)

Broadcast 192.168.1.7  ( - 1  of next multiple which is 8

IP range: 192.168.1.1- 192.168.1.6 

To conclude in order for an address to be valid it needs to be within the usable IP  or subnet range? 

[Joseph W. Doherty · Accepted solution]

If you mean by "to be valid address", a host address, correct.  But every IP is valid, you need to further add, valid for what.

For example, you correctly dissected 192.168.1.5/30, but if you do the same for 192.168.1.5/28, not only do you have more host IPs, but 192.168.1.4 and 192.168.1.7 are now part of those valid host IPs, i.e. no longer the network number or network broadcast IPs.  If you do 192.168.1.5/29, the network IP changes, but the network broadcast is the same as for 192.168.1.5/30.

You have it right when you figure out what the total IP range is, then set aside the first and last IPs of that range.

[AthertonB9]

yes host address

[AthertonB9]

are all devices on a network workstations, switches, printers, laptops, routers given  host ip addresses out of the range  ?

Also is a bad mask  or invalid address any address that falls outside of the available host Ip address?

[Joseph W. Doherty · Accepted solution]

"are all devices on a network workstations, switches, printers, laptops, routers given  host ip addresses out of the range  ?"

Generally, yes.  However, keep in mind, range sizes can vary much based on number of host IPs you need for your purpose.

"Also is a bad mask  or invalid address any address that falls outside of the available host Ip address?"

First, keep in mind, it's generally not the mask that's invalid, but the IP with that mask.  Again, in the example above, some IP addresses vary in their possible uses by what mask is being used.  (The issue presented in your OP, i.e. how the same IP was, or was not, valid [as a host IP] based on the mask [which determines the address range].)

On some devices, you can get same or similar errors based on conflicts on address ranges on interfaces they overlap.  For example, consider how ranges of addresses can overlap, e.g. 192.168.1.1/24 address range overlaps with 192.168.1.128/30.  This also holds for such an overlap anywhere in the "same" overall network, although if overlap not assigned on the same device, device will likely NOT provide an error.

However, in networks, we often "advertise" summary address ranges where overlap is permitted.  I.e. within the same overall network, as just mentioned, you cannot have the individual networks 192.168.1.0/24 and 192.168.1.128/30 but you can have the network 192.168.1.128/30 and the summary or aggregate 192.168.1.0/24.  (If all of this is last information is new, it can be confusing, but eventually you'll want to understand it too.  It's also extremely powerful/necessary.)

[AthertonB9]

Thank you for your much appreciated support and astute examination of the question. I would say I am more clearer and confident than when I first started of, with that said I think this concludes our discourse.