📘Cisco DevNet Associate (200-901 DEVASC)
What is an IP Address?
An IP address is a unique identifier assigned to a device on a network. It allows devices to find and communicate with each other. Think of it as the device’s network identity.
There are two types of IP addresses:
- IPv4 – 32-bit address, written as four decimal numbers separated by dots.
Example:192.168.1.10 - IPv6 – 128-bit address, written as eight groups of hexadecimal numbers separated by colons.
Example:2001:0db8:85a3:0000:0000:8a2e:0370:7334
Key Points for the Exam:
- Every device on a network must have a unique IP address.
- IPv4 is more common, but IPv6 is becoming necessary due to IPv4 exhaustion.
- IP addresses are assigned statically (manual configuration) or dynamically (via DHCP).
2. Subnet Masks / Prefixes
What is a Subnet Mask?
A subnet mask is used to separate the network part and the host part of an IP address. It tells the device which addresses are local and which are remote.
IPv4 Example:
- IP:
192.168.1.10 - Subnet mask:
255.255.255.0
Here:
192.168.1is the network portion.10is the host portion
Prefix Notation
Instead of writing 255.255.255.0, you can use CIDR notation with a prefix: /24
/24= 24 bits for the network, 8 bits for hosts
Common Subnet Masks / Prefixes:
| Prefix | Subnet Mask | # of Hosts |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /16 | 255.255.0.0 | 65,534 |
Key Points for the Exam:
- Subnet masks are crucial for routing and network segmentation.
- Prefix length is a shortcut to define how big the network is.
3. Gateways
What is a Gateway?
A gateway is a device (usually a router) that connects a local network to other networks. It is the default route a device uses to send traffic outside its own network.
Example:
- Device IP:
192.168.1.10/24 - Gateway:
192.168.1.1
If the device wants to reach 8.8.8.8 (outside the local network), it sends traffic to the gateway, which forwards it toward the destination.
Key Points for the Exam:
- Default gateway must be in the same subnet as the device.
- Without a gateway, a device cannot communicate outside its own network.
4. Routes
What is a Route?
A route tells a device where to send network traffic based on the destination IP address. Routes are stored in a routing table.
- Direct route: Traffic for devices in the same subnet goes directly.
- Default route:
0.0.0.0/0in IPv4 (or::/0in IPv6) — used when no specific route exists, usually points to the gateway. - Static route: Manually configured route.
- Dynamic route: Learned automatically via routing protocols (e.g., OSPF, EIGRP).
Example Routing Table Entry:
| Destination | Subnet Mask | Next Hop / Gateway |
|---|---|---|
| 192.168.1.0 | 255.255.255.0 | Directly Connected |
| 0.0.0.0 | 0.0.0.0 | 192.168.1.1 |
Traffic to 192.168.1.50 → sent directly
Traffic to 8.8.8.8 → sent to 192.168.1.1 (gateway)
Key Points for the Exam:
- Routing ensures packets reach devices beyond the local network.
- Default route is essential for Internet access.
- Routing table entries can be viewed using
show ip route(Cisco CLI).
5. How These Components Work Together
- Device has an IP and subnet mask → knows which devices are local.
- Device sends local traffic directly → no gateway needed.
- Device sends traffic to other networks → uses the gateway.
- Routing table determines the next hop → either direct or via a gateway.
Example Flow:
- Device:
192.168.1.10/24 - Gateway:
192.168.1.1 - Destination:
10.0.0.5
Steps:
- Check subnet →
10.0.0.5is outside192.168.1.0/24 - Look in routing table → default route exists → send traffic to
192.168.1.1 - Gateway forwards traffic toward
10.0.0.5
6. Exam Tips
- Know the difference between IP address, subnet, gateway, and route.
- Be able to calculate network, broadcast, and host ranges from an IP and prefix.
- Understand how devices determine whether traffic is local or needs a gateway.
- Know commands like
ipconfig,ping,tracert, andshow ip routefor troubleshooting. - Remember: Default gateway = your path to other networks, Subnet mask = defines network size, Routing table = guides packets to their destination.
