:root {
  --bg: #f6f7f9;
  --panel: #ffffff;
  --text: #1c2333;
  --muted: #5b6472;
  --border: #dfe3e8;
  --accent: #2f6fed;
  --accent-bg: #e8f0fe;
  --error-bg: #fdecec;
  --error-text: #9b1c1c;
  --bit-net: #2f6fed;
  --bit-net-bg: #e8f0fe;
  --bit-borrow: #d9822b;
  --bit-borrow-bg: #fbeadb;
  --bit-host: #5b6472;
  --bit-host-bg: #eef0f3;
  --radius: 8px;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: "Segoe UI", system-ui, -apple-system, sans-serif;
  line-height: 1.5;
}

header.site {
  background: var(--panel);
  border-bottom: 1px solid var(--border);
  padding: 0 1.5rem;
}

.site-inner {
  max-width: 900px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  gap: 2rem;
  flex-wrap: wrap;
}

.site h1 {
  font-size: 1.15rem;
  margin: 0.9rem 0;
  white-space: nowrap;
}

nav.tabs {
  display: flex;
  gap: 0.25rem;
}

nav.tabs a {
  display: inline-block;
  padding: 0.9rem 0.9rem;
  text-decoration: none;
  color: var(--muted);
  border-bottom: 3px solid transparent;
  font-size: 0.95rem;
}

nav.tabs a.active,
nav.tabs a:hover {
  color: var(--accent);
  border-bottom-color: var(--accent);
}

main {
  max-width: 900px;
  margin: 0 auto;
  padding: 1.5rem;
}

.panel {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.25rem 1.5rem;
  margin-bottom: 1.25rem;
}

h2 {
  margin-top: 0;
}

label {
  display: block;
  font-weight: 600;
  margin: 0.75rem 0 0.25rem;
  font-size: 0.9rem;
}

input[type="text"],
input[type="number"] {
  width: 100%;
  padding: 0.5rem 0.6rem;
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 1rem;
  font-family: inherit;
}

.radio-row {
  display: flex;
  gap: 1.5rem;
  align-items: center;
  margin-top: 0.5rem;
}

.radio-row label {
  font-weight: 400;
  margin: 0;
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

button,
.btn {
  background: var(--accent);
  color: #fff;
  border: none;
  padding: 0.6rem 1.2rem;
  border-radius: 6px;
  font-size: 1rem;
  cursor: pointer;
  margin-top: 1rem;
}

button.secondary {
  background: transparent;
  color: var(--accent);
  border: 1px solid var(--accent);
  padding: 0.35rem 0.7rem;
  font-size: 0.85rem;
  margin-top: 0;
}

.error {
  background: var(--error-bg);
  color: var(--error-text);
  border: 1px solid #f3c2c2;
  border-radius: var(--radius);
  padding: 0.9rem 1.1rem;
  margin-bottom: 1.25rem;
}

.steps {
  list-style: none;
  padding: 0;
  margin: 0;
}

.steps li {
  padding: 0.7rem 0;
  border-top: 1px solid var(--border);
}

.steps li:first-child {
  border-top: none;
}

.steps .step-title {
  font-weight: 600;
  margin-bottom: 0.2rem;
}

.steps .step-text {
  color: var(--muted);
  font-size: 0.95rem;
}

table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 0.5rem;
  font-size: 0.9rem;
}

th, td {
  text-align: left;
  padding: 0.5rem 0.6rem;
  border-bottom: 1px solid var(--border);
  white-space: nowrap;
}

th {
  color: var(--muted);
  font-weight: 600;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.03em;
}

.bitbox {
  font-family: "Cascadia Code", "Consolas", monospace;
  display: inline-flex;
  gap: 0.3rem;
  font-size: 0.85rem;
}

.bitbox .octet {
  display: inline-flex;
  border-radius: 4px;
  overflow: hidden;
  border: 1px solid var(--border);
}

.bitbox .bit {
  padding: 0.15rem 0.15rem;
  width: 1.05em;
  text-align: center;
}

.bit.n { color: var(--bit-net); background: var(--bit-net-bg); }
.bit.b { color: var(--bit-borrow); background: var(--bit-borrow-bg); font-weight: 700; }
.bit.h { color: var(--bit-host); background: var(--bit-host-bg); }

.legend {
  display: flex;
  gap: 1.2rem;
  font-size: 0.85rem;
  color: var(--muted);
  margin-top: 0.6rem;
  flex-wrap: wrap;
}

.legend .swatch {
  display: inline-block;
  width: 0.8em;
  height: 0.8em;
  border-radius: 2px;
  margin-right: 0.3rem;
  vertical-align: -0.05em;
}

.legend .swatch.n { background: var(--bit-net-bg); border: 1px solid var(--bit-net); }
.legend .swatch.b { background: var(--bit-borrow-bg); border: 1px solid var(--bit-borrow); }
.legend .swatch.h { background: var(--bit-host-bg); border: 1px solid var(--bit-host); }

.vlsm-row {
  display: grid;
  grid-template-columns: 2fr 1fr auto;
  gap: 0.6rem;
  align-items: end;
  margin-top: 0.6rem;
}

.hint {
  color: var(--muted);
  font-size: 0.85rem;
  margin-top: 0.3rem;
}

code {
  background: var(--bit-host-bg);
  padding: 0.1rem 0.35rem;
  border-radius: 4px;
  font-size: 0.9em;
}

footer {
  text-align: center;
  color: var(--muted);
  font-size: 0.8rem;
  padding: 2rem 1rem;
}