Let's analyze each mask and check if it can be a valid CIDR mask. ### **CIDR Mask Requirements** A valid CIDR subnet mask must have all 1s on the left followed by all 0s on the right, with no mixing (no 0s between 1s). --- ### 1. `255.225.0.0` Let's write the octets in binary: - 255 = `11111111` - 225 = `11100001` - 0 = `00000000` - 0 = `00000000` So: ``` 11111111.11100001.00000000.00000000 ``` See the second octet: `11100001` There are 3 ones, then a zero, then four more ones. This is **not contiguous**. In a valid mask, after the first zero appears, there should be only zeros. **Therefore: `255.225.0.0` CANNOT be a CIDR mask.** --- ### 2. `255.192.0.0` - 255 = `11111111` - 192 = `11000000` - 0 = `00000000` - 0 = `00000000` ``` 11111111.11000000.00000000.00000000 ``` All 1s followed by all 0s. **Valid.** --- ### 3. `255.255.255.6` - 255 = `11111111` - 255 = `11111111` - 255 = `11111111` - 6 = `00000110` ``` 11111111.11111111.11111111.00000110 ``` The last octet: `00000110` There are zeros, then ones, then zeros. **Not contiguous.** **Therefore: `255.255.255.6` CANNOT be a CIDR mask.** --- ## **Summary Table** | Mask | Valid CIDR Mask? | Why? | |---------------------|:----------------:|-----------------------------------------------------------| | 255.225.0.0 | NO | Bits are not contiguous (1s and 0s mixed in 2nd octet) | | 255.192.0.0 | YES | All 1s followed by all 0s | | 255.255.255.6 | NO | Bits are not contiguous (1s and 0s mixed in last octet) | --- ## **Final Answer** The following **cannot be a mask in CIDR** (and **why**): - **255.225.0.0** — The binary form (`11100001`) has non-contiguous 1s and 0s. - **255.255.255.6** — The binary form (`00000110`) has non-contiguous 0s and 1s. **CIDR masks must have all 1s on the left and all 0s on the right, with no mixing.**