Network Basics CTF

#01 Loopback Address

50 pts OPEN ▼

Every machine has a special IP address that refers to itself. When you want to test networking on the local machine without sending packets to the network, you use this address. It is part of the 127.0.0.0/8 block and resolves to the hostname localhost.

# What IP address is used for loopback?
$ ping ________
PING localhost (________): 56 bytes of data
64 bytes from ________: icmp_seq=0 ttl=64

Think of the very first address in the 127.x.x.x range.

Flag Captured

NET{l00pb4ck_1s_h0m3}

#02 Subnet Hosts

75 pts OPEN ▼

A /24 subnet is one of the most common network sizes. Given the network address 192.168.1.0/24, how many usable host addresses are available? Remember: the network address and broadcast address cannot be assigned to hosts.

# Network: 192.168.1.0/24
# Subnet mask: 255.255.255.0
# How many usable host IPs?
Total IPs = 2^(32-24) = ?
Usable = Total - 2 = ?

2^8 = 256 total addresses. Subtract network + broadcast.

Flag Captured

NET{254_h0sts_p3r_sl4sh24}

#03 ARP Protocol

75 pts OPEN ▼

Before sending a packet to another device on the same network, your machine needs to know its hardware address. A specific protocol is used to resolve IP addresses to MAC addresses. This protocol is also the target of spoofing attacks used in MITM attacks.

# Capture shows a broadcast request:
Who has 192.168.1.1? Tell 192.168.1.105
# Protocol name? (3 letters)

Address Resolution Protocol. The command to view its table is `arp -a`.

Flag Captured

NET{4rp_p0is0n_v3ct0r}

#04 Broadcast Address

100 pts OPEN ▼

The broadcast address is used to send packets to all hosts on a network segment. For the network 192.168.1.0/24, what is the broadcast address? It is the last address in the range.

# Network: 192.168.1.0
# First host: 192.168.1.1
# Last host: 192.168.1.254
# Broadcast: ?

All host bits set to 1. In /24, the last octet becomes 255.

Flag Captured

NET{br04dc4st_4ll_h0sts}

#05 CIDR Notation

150 pts OPEN ▼

CIDR (Classless Inter-Domain Routing) notation expresses subnet masks as a prefix length. Convert the subnet mask 255.255.255.0 to its CIDR prefix number. Count the number of consecutive 1-bits in the binary representation.

# Convert: 255.255.255.0 → /??
255 = 11111111 (8 bits)
255 = 11111111 (8 bits)
255 = 11111111 (8 bits)
0 = 00000000 (0 bits)
# Total 1-bits = ?

Count all the 1-bits: 8+8+8+0 = ?

Flag Captured

NET{c1dr_m4st3r_sl4sh24}