The seven layers of the OSI model.
23
The firewall blocks it. Almost all router have telnet port blocked because it's considered dangerous.
OSI
TELNET operates at Application Layer in TCP/IP Model. It's an Application to establish a connection to TCP Port Number 23, where a Telnet server application (telnetd) is listening.
TELNET
an upper-layer, connection-oriented protocol or service
FTP and Telnet are considered OSI layer 7 protocols. If you are blocking the service, then layer 3 or 4 (network or transport) is responsible.
The OSI Model stands for Open Systems Interconnection. The answer to the question which layer of the OSI model are layers capable of functioning is at all layers.
Telnet is an application layer protocol (Layer 7). Its functionality is similar to any of the layer 7 protocols, including DHCP, DNS, FTP, etc.
application layer
application -top of osi model physical- bottom of osi model